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>
glslparsertest: Fix parsing of GL_SHADING_LANGUAGE_VERSION in GLES.

In desktop GL, glGetString(GL_SHADING_LANGUAGE_VERSION) returns a
string that begins with the supported version number.  But in GLES,
the version number is preceded by the text "OpenGL ES GLSL ES ".  This
was causing glslparsertest to fail to parse the implementation's
supported GLSL version, and that in turn caused parse_glsl_version()
function to access uninitialized memory.

This patch splits parse_glsl_version() into two functions:
parse_glsl_version_number(), which parses the bare version number, and
parse_glsl_version_string(), which expects the prefix "OpenGL ES GLSL
ES " when compiled for GLES testing.
glslparsertest: Don't require ES compatibility extensions when testing GLES.

glslparsertest should only check for the presence of
GL_ARB_ES{2,3}_compatibility when the test is running under desktop
GL.  GLES never exposes these extensions.
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.
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.