Implement overridable scene render settings

This add a new set of (possible) render settings that can be defined at
the scene level and overridable at the scene layer level.

Once we get workspaces we can either add workspace inbetween scene and
scene layer evaluation. Or to replace layer settings, to avoid extra
confusion to users.

An example of this setting is "samples", as implemented now for the clay
engine.

source/blender/draw/engines/basic/basic_engine.c

@@ -248,7 +248,7 @@ DrawEngineType draw_engine_basic_type = {
 RenderEngineType DRW_engine_viewport_basic_type = {
 	NULL, NULL,
 	BASIC_ENGINE, N_("Basic"), RE_INTERNAL,
-	NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
+	NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
 	&draw_engine_basic_type,
 	{NULL, NULL, NULL}
 };