gs: Test EndPrimitive() functionality.

v2: Use glViewport to draw the two images to separate parts of the
window, rather than drawing each in an fbo and then blitting.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
gs: test all input primitive types.

Verified using the NVIDIA proprietary driver for Linux.
!UPSTREAM: Test core GS inputs on GLSL 1.50

TODO:
- Make this work.