Implement global and per instance shader uniforms.

Adds two keywords to shader language for uniforms:
-'global'
-'instance'

This allows them to reference values outside the material.

scene/scene_string_names.cpp

@@ -202,4 +202,7 @@ SceneStringNames::SceneStringNames() {
 	parameters_base_path = "parameters/";
 
 	tracks_changed = "tracks_changed";
+
+	shader_overrides_group = StaticCString::create("_shader_overrides_group_");
+	shader_overrides_group_active = StaticCString::create("_shader_overrides_group_active_");
 }