Adds back tex_coord array for CoglShader compatibility

This adds back compatibility for CoglShaders that reference the
cogl_tex_coord_in[] or cogl_tex_coord_out[] varyings. Unlike the
previous way this was done this patch maintains the use of layer numbers
for attributes and maintains forwards compatibility by letting shaders
alternatively access the per-layer tex_coord varyings via
cogl_tex_coord%i_in/out defines that index into the array.

cogl/cogl-glsl-shader-private.h

@@ -28,6 +28,7 @@ _cogl_glsl_shader_set_source_with_boilerplate (CoglContext *ctx,
                                                const char *version_string,
                                                GLuint shader_gl_handle,
                                                GLenum shader_gl_type,
+                                               CoglPipeline *pipeline,
                                                GLsizei count_in,
                                                const char **strings_in,
                                                const GLint *lengths_in);