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.
generated_tests: Fix geometry shader tests to use gl_VerticesIn

Commit ff315ae (Add geometry testing to code-generated tests)
erroneously omitted the parentheses when calling
glPositionIn.length().  It also failed to account for the fact that
the .length() method is unavailable in GLSL 1.10.

This patch modifies the code to use gl_VerticesIn instead, since this
is provided by ARB_geometry_shader4, and hence is available regardless
of the GLSL version being tested.

Thanks to Ken Phillis Jr for discovering this bug.

Cc: Ken Phillis Jr <kphillisjr@gmail.com>