There's no need to do that with the nvts and opengl ffp fixed function
fragment pipeline, it's perfectly well defined in GL which one takes
effect. This removes a few more troubles when switching between
shaders and arbfp.

ARB and GLSL don't need that. If a shader backend like atifs or nvts
need it in the future, the shader backend should deal with that rather
than the ffp pipeline.

Half Life 2 uses D3DFMT_X8R8G8B8 for the back buffer, but macos
supports aux buffers only on D3DFMT_A8R8G8B8. I think having aux
buffers is more important right now than having a precise alpha
match.