vout: reduce the pool sizes and make pools local

We no longer need to get pools from the display module. We only have 2 separate pools for the converter and for the filters+blender.

We also don't need to create the converter pool if we know there will be no converter.

This will reduce the amount of memory allocated during playback, especially when there is a converter (20 pictures forced so far).

Tested with and without a converter with multiple filters/blending.

Draft on top of !3987 (merged)