mesa/glsl: stop using GL shader type internally

mesa/glsl: stop using GL shader type internally

Instead use the internal gl_shader_stage enum everywhere. This
makes things more consistent and gets rid of unnecessary
conversions.

Ideally it would be nice to remove the Type field from gl_shader
altogether but currently it is used to differentiate between
gl_shader and gl_shader_program in the ShaderObjects hash table.

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