[Piglit] Make GLSLParser more strict v2

[Dylan Baker]

This is an updated version of a series I sent out previously to make the
GLSLParserTest constructor more strict in what values it allows.

This version is considerably different than v1, largely because it does
a good deal of refactoring before it gets to any behavioral changes.

Patches 1-6 are all groundwork patches that change the interals of the
class, but nothing that a user should notice. Particuarly they remove a
lot of code, and split some very large methods up into smaller methods
that should be easier to manage.

Patches 7-9 are the behavioral changes, they explicitely disallow more
unsupported values for keys. There is room for more strictness, but
these felt like a good balance between being very strict, and not
requiring lots of very specific code.

This is available at my github: 
https://github.com/dcbaker/piglit.git submit/glsl-strict