Always use a non-sRGB texture as the render target so that color
and alpha blending are in sRGB space, matching Flash behavior.
If the surface format requires sRGB, render to an intermediate
linear buffer and copy to the surface as a final render step.
naga is currently lenient about some things which are invalid by
the WGSL spec. The Tint WGSL compiler disagreed, so these shaders
failed to compile in Chrome.