arb_texture_multisample: add test to check sample positions v2

v2: Removed dependency on TextureBarrierNV and ARB_tfb_2, added
comments.
WIP: basic EndPrimitive() test
util: Create generic piglit_draw_rect() functions.  TODO: rework.

This patch adds the following functions to piglit-util-gl-common:
- piglit_draw_rect_generic
- piglit_draw_rect_back_generic
- piglit_draw_rect_z_generic
- piglit_draw_rect_tex_generic

These functions are similar to corresponding functions without the
"_generic" suffix, except that instead of delivering vertex position
and texture coordinates to the vertex shader using glVertexPointer()
and glTexCoordPointer(), they deliver to the generic attributes
PIGLIT_ATTRIB_POS and PIGLIT_ATTRIB_TEX.  These functions use VBO's
and VAO's when available, so they may be used when testing core
contexts.

These functions also work with compatibility contexts and with all
GLES versions.  Since GLES1 doesn't support generic vertex attributes,
we fall back to using glVertexPointer() and glTexCoordPointer() in the
GLES1 case.

When building GLES tests, the behaviour of the new _generic functions
is for all intents and purposes the same as that of the pre-existing
non-generic functions.  So to avoid code duplication, the non-generic
functions simply call their generic equivalents in GLES builds.
WIP: primitive restart draw mode test

TODO: improve initial commentary
WIP: start testing GS in/out types
WIP: fix shader_runner error reporting when vertex data included.
WIP: Basic test of GS input varyings
fixup! WIP: Basic test of GS input varyings
Geometry shaders: add a basic EndPrimitive() test.
WIP: large EndPrimitive test
Test interaction of dead code elimination with "break" statements.

Test written by Vadim Girlin <vadimgirlin@gmail.com>.  I added
explanatory comments.