Observations:
- Entity::renderPass was never set to RENDER_ALL -> can simplify some things
- The initial pass check in RenderObject::render() was constant for each pass
-> All logic that is per-pass-constant can be moved to a renderability pre-check
- Core::overrideStartLayer, Core::overrideEndLayer, Core::rlayer were never used
- Should be possible eventually to prepare & render layers in parallel
I am not sure if the changes in this commit are 100% correct, but layer passes
are still working and the hug looks like it should.
Thinking about it, the overrideRenderPass functionality should never have existed.
The game scripts don't actually use entity_setRenderPass
(which in turn calls Entity::setOverrideRenderPass())
so I might remove that function in a future commit,
together with the rest of the "override" functionality.
