Created attachment 364798 [details]
Problem appears to be fixed in kmscon-8
Not mesa's fault, as far as I can tell.
The fix here is to bump kmscon to 8, but alexxy (not an X11 team member) added kmscon without himself in metadata...
Created attachment 373282 [details]
ebuild that applies some src fixes
This applies the GL_UNPACK_ROW_LENGTH fix and also adds precision specifiers to the fragment shaders. This makes kmscon-7 build (and work) against mesa-10.1.0 for me.