Draft: Rearrange layer renderer layers

This is sort of an intermediate step (/RFC) to make it easier to change the layer renderer.

Details: We have a bunch of comp_layers in the base compositor that need to be communicated down into comp_layer_renderer so rendering can happen. Currently, comp_compositor calls accessors on comp_renderer which calls accessors on comp_layer_renderer. This seems unnecessarily cumbersome and makes it more difficult to move around the layer renderer code. Instead, this MR makes comp_compositor just pass the raw layer info to comp_renderer, which coordinates the setup of comp_layer_renderer as before. It does this in dispatch_graphics, right before invoking the layer renderer, which means it can easily switch to sending the layers to render_gfx instead.