Fix for 2.66a
With help from Daniel Stokes and Mitchell Stokes.
This bug always existed in OSX, but started showing up in Windows and Linux on review (54745 + 54747)
[the patch to enable alpha buffer for all OSs]
A better fix would be to use RAS_IRasterizer::SetAlphaBlend(GPU_BLEND_SOLID);
but I think gpu_verify_alpha_blend() is not switching to SOLID because
GTS.alphablend is GPU_BLEND_SOLID (even though GL_ALPHA_TEST is enabled).
Anyways, this is not something worth tackling now, since in terms of functionality it shouldn't matter.