mesa: Expose GLSL interpolation qualifiers in gl_fragment_program.

mesa: Expose GLSL interpolation qualifiers in gl_fragment_program.

This patch makes GLSL interpolation qualifiers visible to drivers via
the array InterpQualifier[] in gl_fragment_program, so that they can
easily be used by driver back-ends to select the correct interpolation
mode.

Previous to this patch, the GLSL compiler was using the enum
ir_variable_interpolation to represent interpolation types.  Rather
than make a duplicate enum in core mesa to represent the same thing, I
moved the enum into mtypes.h and renamed it to be more consistent with
the other enums defined there.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>