Unify a lot of gles2 vs gl glsl code

Since we used to support hybrid fixed-function + glsl pipelines when
running with OpenGL there were numerous differences in how we handled
codegen and uniform updates between GLES2 and full OpenGL. Now that we
only support end-to-end glsl pipelines this patch can largely unify how
we handle GLES2 and OpenGL.

Most notably we now never use the builtin attribute names. This should
also make it easy for us to support creating strict OpenGL 3.1 contexts
where the builtin names have been removed.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

(cherry picked from commit 2701b93f159bf2d3387cedf2d06fe921ad5641f3)

cogl/cogl-glsl-shader-private.h

@@ -25,6 +25,7 @@
 
 void
 _cogl_glsl_shader_set_source_with_boilerplate (CoglContext *ctx,
+                                               const char *version_string,
                                                GLuint shader_gl_handle,
                                                GLenum shader_gl_type,
                                                int n_tex_coord_attribs,