We can't easily replace texcoords in Vulkan, so we need to do something better for Zink. But since the same thing applies to Bifrost GPUs, let's try to fix this for both at once.
This also fixes some other details I noticed while hacking this together.
This probably needs some more work to ensure that
GL_POINT_SPRITE_COORD_ORIGIN is handled correctly as well, but it's better than nothing.