Test that transform feedback works with non-flat integers.

Commit 0fac0a6 (Modify tests for error checking on "flat" keyword to
match updated specs) adopted the GLSL 1.50 rules for "flat" varyings
for all desktop GL tests (the rationale was that this change likely
reflects a bug fix rather than a deliberate behavioural difference).

The GLSL 1.50 rules allow a vertex shader output to be a non-flat
integer provided that it isn't consumed by the fragment shader.  In
particular, this means that a non-flat integer may be used with
transform feedback.

This test verifies that transform feedback works correctly for all
integral types, even if they are not declared as "flat".
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: test interaction of primitive restart and transform feedback