mesa/glsl: Separate parsing logic from _mesa_get_uniform_location.

The parsing logic is moved to a new function in the GLSL module,
parse_program_resource_name().  This name was chosen because it should
eventually be useful for handling everything that OpenGL 4.3 calls
"program resources" (e.g. uniforms, vertex inputs, fragment outputs,
and transform feedback varyings).

Future patches will make use of this function for linking transform
feedback varyings.

NOTE: This is a candidate for the 9.1 branch.
glsl: Rename uniform_field_visitor to program_resource_visitor.

There's actually nothing uniform-specific in uniform_field_visitor.
It is potentially useful for all kinds of program resources (in
particular, future patches will use it for transform feedback
varyings).

This patch renames it to program_resource_visitor, and clarifies
several comments, to reflect the fact that it is useful for more than
just uniforms.

NOTE: This is a candidate for the 9.1 branch.
glsl: Use parse_program_resource_name to parse transform feedback varyings.

Previously, transform feedback varyings were parsed in an ad-hoc
fashion that wasn't compatible with structs (or array of structs).
This patch makes it use parse_program_resource_name(), which correctly
handles both.

Note that parse_program_resource_name()'s technique for handling
mal-formed input strings is to simply let them through and rely on the
fact that a future name lookup will fail.  Because of this,
tfeedback_decl::init() no longer needs to return a boolean error
code--it always suceeds, and if the input was mal-formed the error
will be detected later.

NOTE: This is a candidate for the 9.1 branch.
glsl: Support transform feedback of varying structs.

Since transform feedback needs to be able to access individual fields
of varying structs, we can no longer match up the arguments to
glTransformFeedbackVaryings() with variables in the vertex shader.

Instead, we build up a hashtable which records information about each
possible name that is a candidate for transform feedback, and then
match up the arguments to glTransformFeedbackVaryings() with the
contents of that hashtable.

Populating the hashtable uses the program_resource_visitor
infrastructure, which so the logic is shared with how we handle
uniforms.

NOTE: This is a candidate for the 9.1 branch.
intel/pre-gen6: Disable EXT_framebuffer_multisample.  TODO: test on Gen5.