Support both core and compat contexts in xfb/overflow-edge-cases.c.

This will allow the test to be adapted for geometry shaders.
gs: Adapt xfb/overflow-edge-cases.c for geometry shaders.

The transform feedback test "overflow-edge-cases" can now be invoked
with a "use_gs" argument, that causes it to use a geometry shader.
Support both core and compat contexts in xfb/intervening-read.c.

This will allow the test to be adapted for geometry shaders.
gs: Adapt xfb/intervening-read.c for geometry shaders.

The transform feedback test "intervening-read" can now be invoked with
a "use_gs" argument, that causes it to use a geometry shader.