mesa: Use gl_shader::Stage instead of gl_shader::Type where possible.

mesa: Use gl_shader::Stage instead of gl_shader::Type where possible.

This reduces confusion since gl_shader::Type is sometimes
GL_SHADER_PROGRAM_MESA but is more frequently
GL_SHADER_{VERTEX,GEOMETRY,FRAGMENT}.  It also has the advantage that
when switching on gl_shader::Stage, the compiler will alert if one of
the possible enum types is unhandled.  Finally, many functions in
src/glsl (especially those dealing with linking) already use
gl_shader_stage to represent pipeline stages; using gl_shader::Stage
in those functions avoids the need for a conversion.

Note: in the process I changed _mesa_write_shader_to_file() so that if
it encounters an unexpected shader stage, it will use a file suffix of
"????" rather than "geom".

Reviewed-by: Brian Paul <brianp@vmware.com>
v2: Split from patch "mesa: Store gl_shader_stage enum in gl_shader objects."

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>