Avoid branch in half2float in gl_compatibility renderer

[clayjohn]

Fixes: #73320

This is a regression from #72914.

For some reason the radeon drivers really don't like the h_e == uint(0x0000) comparison. Replacing it with (h_e >> uint(10)) == uint(0) works though.

Also, when running it through Radeon GPUAnalyzer I noticed that this was actually generating a branch. So I made the code branchless. Logically this code does almost exactly the same thing as the old code (it can now return -0 but couldn't before) but it works on more hardware, so oh well.

CC @JonqsGames you may be interested in this

[akien-mga]

It's a kind of magic

[JonqsGames]

Looks good ! Should we also avoid branch in float2half ?

uint float2half(uint f) {
	uint e = f & uint(0x7f800000);
	return ((f >> uint(16)) & uint(0x8000)) |
			uint((e > uint(0x38000000))) * (((e - uint(0x38000000)) >> uint(13)) & uint(0x7c00)) |
			((f >> uint(13)) & uint(0x03ff));
}

[clayjohn]

Looks good ! Should we also avoid branch in float2half ?

uint float2half(uint f) {
	uint e = f & uint(0x7f800000);
	return ((f >> uint(16)) & uint(0x8000)) |
			uint((e > uint(0x38000000))) * (((e - uint(0x38000000)) >> uint(13)) & uint(0x7c00)) |
			((f >> uint(13)) & uint(0x03ff));
}

We could. I wasn't able to detect any issues with that code on the same device. So my preference is to not touch it. I don't want to risk triggering another driver bug

[akien-mga]

Thanks!