Cleanup PostPorcessManager rendering

- the main goal of this change was to move some state changes outside
  of loops, usually with mip generation.

- for instance bindPostProcessDesciptorSet() must now be issued
  manually (and can be done outside the loop)

- we also don't set the scissor for each pass

- we also move prepareMaterial() outside of getPipelineState(), it's
  now done in PostProcessMaterial::getMaterial().

- We use PostProcessVariant instead of uint8_t everywhere

In the end we have three drawing helpers:

- commitAndRenderFullScreenQuad() which updates a material instance,
  binds the corresponding material and draws a full screen quad.
- renderFullScreenQuad() which just renders a full screen quad.
- renderFullScreenQuadWithScissor() which does the same but scisorred

Additionally, we have the following helpers for getting materials and
instances:

- PostProcessMaterial::getMaterial(): which returns the FMaterial
- PostProcessMaterial::getMaterialInstance(): which is now a helper
  returning a material instance from a PostProcessMaterial or FMaterial

Most of the change is pluming through these API changes.