Implement render-to-FBO

[ammen99]

There should be an option to render the whole screen (not like the workspace streams, but the whole output, as the user would see it) to a texture and then allow plugins to do transforms on that.

A few possible use-cases:

1. Desktop zoom-in animation on startup (not just the current fade)
2. Rotation animation like the one Android/iOS have
3. Zoom support - the ability to zoom a part of the screen around the cursor.
4. All kinds of post-processing - color transforms (inversion, black-and-white, others), motion blur and what not.

This would actually be relatively easy to implement. We'd have to add a new type of hook - postprocessing hook. Each one should have a source framebuffer and a target framebuffer, so that they can be composed together - for example, we should have the ability to zoom and have a black-and-white output filter at the same time.