workaround specific Mesa bugs

* Mesa's "Software Rasterizer" renderer has bugs that
prevent depth peeling. all related ctests fail. disable
this feature.

* Mesa's "llvmpipe" has bugs that prevent depth peeling.
some of the ctests fail. disable this feature.

* Mesa version 7 "Software Rasterizer" GL_ARB_draw_buffers
has a bug that leaves FBO in a perpetually incomplete state,
disable FBO for this renderer.

Change-Id: I46f4f175fbd634a67eeeff904e19ef195494c1ff
Merge topic 'workaround_specific_mesa_bugs' into master

d9d8d222 workaround specific Mesa bugs