mesa: Clean up nomenclature for pipeline stages.

mesa: Clean up nomenclature for pipeline stages.

Previously, we had an enum called gl_shader_type which represented
pipeline stages in the order they occur in the pipeline
(i.e. MESA_SHADER_VERTEX=0, MESA_SHADER_GEOMETRY=1, etc), and several
inconsistently named functions for converting between it and other
representations:

- _mesa_shader_type_to_string: gl_shader_type -> string
- _mesa_shader_type_to_index: GLenum (GL_*_SHADER) -> gl_shader_type
- _mesa_program_target_to_index: GLenum (GL_*_PROGRAM) -> gl_shader_type
- _mesa_shader_enum_to_string: GLenum (GL_*_{SHADER,PROGRAM}) -> string

This patch tries to clean things up so that we use more consistent
terminology: the enum is now called gl_shader_stage (to emphasize that
it is in the order of pipeline stages), and the conversion functions are:

- _mesa_shader_stage_to_string: gl_shader_stage -> string
- _mesa_shader_enum_to_shader_stage: GLenum (GL_*_SHADER) -> gl_shader_stage
- _mesa_program_enum_to_shader_stage: GLenum (GL_*_PROGRAM) -> gl_shader_stage
- _mesa_progshader_enum_to_string: GLenum (GL_*_{SHADER,PROGRAM}) -> string

In addition, MESA_SHADER_TYPES has been renamed to MESA_SHADER_STAGES,
for consistency with the new name for the enum.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
v2: Also rename the "target" field of _mesa_glsl_parse_state and the
"target" parameter of _mesa_shader_stage_to_string to "stage".

Reviewed-by: Brian Paul <brianp@vmware.com>