[FIX] Adopt top-left texture origin

[slimbuck]

Fixes: #3322

This PR changes the orientation of textures in the engine from bottom-left to top-left, bringing it inline with WebGL and glTF.

Specifically, the following changes are made:

• stop flipping png/jpg textures vertically at load time
• stop inverting glTF texture coordinates at load time
• start inverting JSON model texture coordinates at load time
• invert dynamically-generated texture coordinates for:
  • text
  • images
  • particles
  • sprites
• invert texture coordinates of built-in shapes
• update the basis texture engine example so it's not ugliest of all

NOTE: the following shader chunks have been changed in this PR:

• particle.frag
• particle_cpu.vert
• particle_normalMap.frag
• startNineSliced.frag
• startNiceSlicedTiled.frag

Users who have custom versions of these chunks will have to update these accordingly.

[Maksims]

This change has the potential to break a lot of advanced users projects who render their own textures and use them in shaders.
Could this change be added to respect backwards compatibility, and have a project default setting to use the bottom-left coordinate system. And for the new projects, flipping the switch?

So it won't break any existing projects and ensure a smooth transition, which can be done by the developer by choice.

[mvaligursky]

looks great to me, glad to have this sorted out!

[slimbuck]

Hi @Maksims ,

Yes you're right. This change has the potential to impact projects, especially ones that dynamically generate texture coordinates or have custom chunks generating UVs. Fortunately in most cases the fix is simple, place uv.y = 1.0 - uv.y; at the end.

Keeping backwards compatibility would be difficult in the engine (think different chunks etc) and would leave us having to maintain quite a complicated global option. Over time we'd expect to see bugs creep in and ultimately we decided that making a clean break is the best approach here.

Also, with this change in place we can finally support new features that weren't possible before (glTF texture transform, glTF quantising, gltf compressed textures). If we were to keep backwards compatibility those features would also be compromised.

Along with this change we have also updated the backend to stop flipping compressed textures. When this PR goes live, users will get an alert in the editor saying textures need recompressing. A single click will recompress all textures in the project.

This is a big change which might be a little painful, but we are preparing to make the transition hopefully as smooth as possible!