Set process priority of framework window to high when focused

[peppy]

This is something we do on osu!stable and has large gains (for some users) on desktop systems, especially when shared with other heavy applications.

Tested on windows and macOS. Windows works as expected, macOS fails due to lack of permissions. Would appreciate linux testing.

If deemed more appropriate, can be moved to WindowsGameHost.

[Susko3]

Have you considered how this may affect headless testing? HeadlessGameHost derives from DesktopGameHost and is considered to be active by default. Tests probably shouldn't be using high priority.

[bdach]

I'm not sure it realistically matters until proven otherwise. Do you have some specific concern in mind?

[Susko3]

My concerns mostly come from the tone of the documentation: "Use extreme care when specifying High for the process's priority class [...]"
And this affecting headless tests being a "hidden" change.

I've just run the full framework test suite and haven't noticed any slowdowns on my machine, so it's probably fine.

[bdach]

Linux also fails on lack of permissions (only when attempting to set to high).

System.ComponentModel.Win32Exception (13): Permission denied
   at System.Diagnostics.Process.set_PriorityClassCore(ProcessPriorityClass value)
   at System.Diagnostics.Process.set_PriorityClass(ProcessPriorityClass value)
   at osu.Framework.Platform.DesktopGameHost.setGamePriority(Boolean active) in /home/dachb/Documents/opensource/osu-framework/osu.Framework/Platform/DesktopGameHost.cs:line 133

[bdach]

Seems probably fine to me. Let's see how this pans out.

@peppy your call on whether you want to address the tests concern brought up above.

[peppy]

i think it might be best to, but hesitant because I'm not sure how to structure it without thinking for more than a few seconds.

[bdach]

Probably make setGamePriority() virtual and suppress in the headless impl? Bit dirty but at this point... stuff like that already exists, i.e.

osu-framework/osu.Framework/Platform/HeadlessGameHost.cs

Lines 94 to 97 in 4fe6df7

	protected override void DrawFrame()
	{
	//we can't draw.
	}

That, or move to WindowsGameHost as proposed in the OP, as it sure looks like it's not gonna work anywhere else anyway.