Test accuracy of dFdx and dFdy functions.

Mesa/i965 currently (as of commit 1cc3b90) has an inaccurate
implementation of dFdy() which produces the same results for every
pixel in an aligned 2x2 block.  This piglit test exposes the
inaccuracy.
gs: Test core GS inputs on GLSL 1.50
gs: Test corner case for triangle strip ordering with primitive restart.

Verified using the NVIDIA proprietary driver for linux.