Make rendering pipeline more flexible

[ffreyer]

Currently the rendering pipeline in GLMakie is rather strict. As a result buffers needed for ssao and fxaa are always created and post processing always runs. As things get more complex it would be good to make this more flexible.

It should be posible to disable fxaa, ssao, etc for the full scene. This should cause less framebuffers to be created (i.e. only those that are actually needed) and the rendering pipeline to be shorter. It should also cause less framebuffers to be written to in fragment_output.frag. Potentially we also want to change which shaders are used in the primary render, e.g. for deferred shading.