glsl: Rework builtin_variables.cpp to reduce code duplication.

Previously, we had a separate function for setting up the built-in
variables for each combination of shader stage and GLSL version
(e.g. generate_110_vs_variables to generate the built-in variables for
GLSL 1.10 vertex shaders).  The functions called each other in ad-hoc
ways, leading to unexpected inconsistencies (for example,
generate_120_fs_variables was called for GLSL versions 1.20 and above,
but generate_130_fs_variables was called only for GLSL version 1.30).
In addition, it led to a lot of code duplication, since many varyings
had to be duplicated in both the FS and VS code paths.  With the
advent of geometry shaders (and later, tessellation control and
tessellation evaluation shaders), this code duplication was going to
get a lot worse.

So this patch reworks things so that instead of having a separate
function for each shader type and GLSL version, we have a function for
constants, one for uniforms, one for varyings, and one for the special
variables that are specific to each shader type.

In addition, we use a class, builtin_variable_generator, to keep track
of the instruction exec_list, the GLSL parse state, commonly-used
types, and a few other variables, so that we don't have to pass them
around as function arguments.  This makes the code a lot more compact.

I've also gone ahead and introduced the variables needed for the
{ARB,EXT}_geometry_shader4 extension; GLSL 1.50-style geometry shader
inputs are not fully supported yet.

This patch doesn't introduce any functional changes (since geometry
shaders haven't been enabled yet).  Future patches will correct a few
bugs that were found during the refactoring process.
glsl ES: Fix magnitude of MaxVertexUniformVectors.

Previously, we set it equal to MaxVertexUniformComponents.  It should
be MaxVertexUniformComponents / 4.
glsl: Make gl_TexCoord compatibility-only

gl_TexCoord was deprecated in GLSL 1.30.  In GLSL 1.40 it was marked
as ARB_compatibility-only, and in GLSL 1.50 and above it was marked as
only appearing in the compatibility profile.  It has never appeared in
GLSL ES.

However, Mesa erroneously included it in all desktop versions of GLSL,
even versions 1.40 and 1.50 (which do not currently support the
compatibility profile).  This patch makes gl_TexCoord available in the
compatibility profile (and GLSL versions 1.30 and prior) only.