Upgraded font-weight

[MexUK]

This PR upgrades support for font-weight in .rcss files:

font-weight: (uint:0-0xFFFF)
and
font-weight: thin|light|normal|medium|bold|black

The PR supports 2 ways to load weights for fonts:

Rml::LoadFontFace("assets/OpenSans.ttf", false, 300); // Only load weight 300 from this font file.
or
Rml::LoadFontFace("assets/OpenSans.ttf", false); // Load all weights from this font file. (The third arg defaults to 0).

The PR also tries to match faces by the nearest weight, after trying to match by a direct weight.

[mikke89]

Thank you! This is awesome, a very welcome addition.

I have some suggestions here after looking over it, let me know what you think.

Haven't tested it properly yet, I see the PR is failing many of the unit tests. Let me know if I can be of help for fixing them.

[MexUK]

I've made a commit with the requested code changes.
I'll try and look into the unit tests soon.
If anything else needs changing, just let me know.

[mikke89]

Thanks for continuing to work on this!

Can I ask about some use cases:

• How do we handle the case when the font face does not contain the desired weight? Do we load it at all?
• If we still load it, what should the font weight be registered as?

[mikke89]

I tested this PR more thoroughly now. Appreciate your continued work on this! Sorry about my reviews coming a bit in chunks, I have gone over everything now though.

Currently, the samples don't open correctly since all included fonts fail to load. It seems to fail whenever the font file does not provide anything for FT_Get_MM_Var, which seems common for all single-weight fonts. A quick fix is to continue after GetFaceInstanceIndexForWeight regardless, when the weight is auto. However, this is not sufficient for all cases. See the following test code for loading fonts in the sample shell:

void Shell::LoadFonts(const char* directory)
{
	using Rml::Style::FontWeight;
	
	struct FontFace {
		Rml::String filename;
		bool fallback_face;
		FontWeight weight;
	};
	FontFace font_faces[] = {
		{ "OpenSans-VariableWidth.ttf",    false, FontWeight::Auto },  // Loads weights [300, 400, 600, 700, 800, 300, 400, 600, 700, 800] (loads faces twice!)
		{ "OpenSans-VariableWidth.ttf",    false, FontWeight(800) },   // Loads 800 (correct)
		{ "OpenSans-VariableWidth.ttf",    false, FontWeight::Light }, // Loads 300 (correct)
		
		{ "OpenSans-Regular.ttf",    false,   FontWeight::Auto }, // Fails to load, with basic fix it loads as 400 (correct)
		{ "OpenSans-Bold.ttf",       false,   FontWeight::Auto }, // Fails to load, with basic fix it loads as 700 (correct)
		{ "OpenSans-ExtraBold.ttf",  false,   FontWeight::Auto }, // Fails to load, with basic fix it loads as 400 (wrong)
		{ "OpenSans-Light.ttf",      false,   FontWeight::Auto }, // Fails to load, with basic fix it loads as 400 (wrong)
		
		{ "OpenSans-Regular.ttf",    false,   FontWeight::Normal }, // Fails to load (ideally this would load correctly and be registered as 400)
		{ "OpenSans-Bold.ttf",       false,   FontWeight::Bold },   // Fails to load (ideally this would load correctly and be registered as 700)
		{ "OpenSans-ExtraBold.ttf",  false,   FontWeight::Bold },   // Fails to load (ideally this would load correctly and be registered as 700)
		{ "OpenSans-ExtraBold.ttf",  false,   FontWeight::Black },  // Fails to load (ideally this would load correctly and be registered as 900)
		
		{ "LatoLatin-Regular.ttf",    false, FontWeight::Auto }, // Fails to load, with basic fix it loads as 400 (correct)
		{ "LatoLatin-Italic.ttf",     false, FontWeight::Auto }, // Fails to load, with basic fix it loads as 400 (correct)
		{ "LatoLatin-Bold.ttf",       false, FontWeight::Auto }, // Fails to load, with basic fix it loads as 700 (correct)
		{ "LatoLatin-BoldItalic.ttf", false, FontWeight::Auto }, // Fails to load, with basic fix it loads as 700 (correct)
		{ "NotoEmoji-Regular.ttf",    true , FontWeight::Auto }, // Fails to load, with basic fix it loads as 400 (correct)
	};

	for (const FontFace& face : font_faces)
	{
		Rml::LoadFontFace(Rml::String(directory) + face.filename, face.fallback_face, face.weight);
	}
}

What we want is the ability to override the font weight if it registers as the wrong one.

Thus, I propose this functionality for the weight_to_load parameter:

• If the font provides multiple font weights, then the weight parameter works like now: Loads only the appropriate weight and fails if it cannot be found.
• On the other hand, if the font face contains only a single face (whether or not it provides MM_var), then the weight parameter overrides whatever is auto-detected.
• This would also ensure backward compatibility, currently it fails loading the debugger plugin for this reason.

If you agree, we should probably change the weight_to_load parameter back to weight again, since it now has two meanings, one of them not having anything to do with loading. This behavior should be noted in the function calls in Core.h .

[mikke89]

Hey. I'm just wondering if you intend to keep working on this PR? We are getting close now, but otherwise I can offer to fix up the last bits so we can get it merged. Cheers!

[MexUK]

Hey, apologies for not finishing the code changes, I've been very busy recently. Thanks for offering to finish them, that would be awesome if you could.

[mikke89]

No worries! I'll give it a go then.

[mikke89]

Hey!

I fixed some of the issues and changes discussed, in addition to some general refactoring. See the commit messages for details. Could you test this on your end? Let me know if you have any suggestions for improvements.