i965 gen6: Turn on transform feedback extension.
i965 gen6: Turn on transform feedback extension.

This patch advertises support for EXT_transform_feedback on Intel
Gen6.

Since transform feedback support is not completely finished yet, for
now we only advertise support for it when MESA_GL_VERSION_OVERRIDE is
3.0 or greater (since transform feedback is required by GL version
3.0).

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

jbarczak/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,djreep81/glsl-optimizer,mapbox/glsl-optimizer,benaadams/glsl-optimizer,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,zeux/glsl-optimizer,mcanthony/glsl-optimizer,djreep81/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,bkaradzic/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,benaadams/glsl-optimizer,tokyovigilante/glsl-optimizer,tokyovigilante/glsl-optimizer,bkaradzic/glsl-optimizer,bkaradzic/glsl-optimizer,tokyovigilante/glsl-optimizer,mapbox/glsl-optimizer,metora/MesaGLSLCompiler,bkaradzic/glsl-optimizer,mapbox/glsl-optimizer,zeux/glsl-optimizer,zeux/glsl-optimizer,dellis1972/glsl-optimizer,metora/MesaGLSLCompiler,zz85/glsl-optimizer,tokyovigilante/glsl-optimizer,djreep81/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer,wolf96/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,dellis1972/glsl-optimizer,zz85/glsl-optimizer,mapbox/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,jbarczak/glsl-optimizer,jbarczak/glsl-optimizer,djreep81/glsl-optimizer,metora/MesaGLSLCompiler,wolf96/glsl-optimizer