The current approach was to pair each FBO with its use, "semantically".
The intent was to minimize the number of "reallocations" that would be
required if the number of passes changed dynamically (e.g. as the
renderer options changed). However, this is not only an unrealistic
design goal to optimize for (users can use separate pl_renderers for
wildly different purposes, and for a single conceptual video stream it
doesn't really matter), but also, it gets in the way of a planned
refactor I have concerning `struct img`.

Change this to make all FBOs dynamically allocated. The current
implementation simply uses a counter, but a more advanced implementation
could use a pool of textures and find ones that have matching sizes
before recreating ones that don't. The API shouldn't change as a result
of this.