glslparsertest: Avoid uninitialized vars in parse_glsl_version_number.

Previously, if the user specified an ill-formed GLSL version number
(or the implementation supplied an ill-formed number in its response
to glGetString(GL_SHADING_LANGUAGE_VERSION)), glslparsertest would
access uninitialized variables, resulting in unpredictable (and often
confusing) behaviour.

With this patch, glslparser test accepts version numbers either of the
form "<int>" or "<int>.<int>".  Ill-formed version numbers lead to a
test failure.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Tom Gall <tom.gall@linaro.org>

v2: Add comment explaining the use of sscanf(...) == 0.
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.