Revert "GL3 mode seems mostly fine on Radeon with recent Mesa, so use it."

This reverts commit 8f90175fc2b17a521005708f819b519f5e4b3169.

Turns out that we're better off with GLDRV_MESA for the moment.
Although we can use GLSL 1.3 without any performance impact,
it doesn't seem worth doing so.