Specifically, the Gallium r600 driver reports "X.Org" for vendor and a string
of the form "Gallium 0.4 on AMD RV730" for the renderer.

The floating point formats from ARB_texture_rg are only defined when
ARB_texture_float is also supported. It would have been nice if the formats
table itself was able to express this, but as far as I'm aware this is only an
issue for ARB_texture_rg, so it's probably not worth changing.

This is consistent with device_setup_fullscreen_window(), and ddraw actually
has a test for this in ddrawmodes.c. (Although it doesn't actually use
device_setup_fullscreen_window() until the next patch.)