Most of the renderpasses in EEVEE used post-processing on the CPU. For
final image rendering this is sufficient, but when we want to display
the data to the user we don't want to transfer to the CPU to do post
processing to then upload it back to the GPU to display the result.

This patch moves the renderpass postprocessing to a GLSL shader.

This is the first step to do, before we will enable the renderpasses in the viewport.