screencast: unmess the pipewire/wlr split

[columbarius]

Using the on_process event of the PipeWire can create a race condition when a previous started screencast hasn't finished before the next event.

This MR is composed of following parts:

• nuke the racy on_process part
• cleanup some stuff
• intruduce xdpw_buffers
• decouple the actual buffer (xdpw_buffer) from the pw_buffer carrier and swap the buffer from the PipeWire queue after each copy.
• Relax the swap incase the PipeWire queue is lagging behind.

Resolves: #182

[columbarius]

This solves the current race condition quite nicely. Additionally this design won't hinder the adoption of dmabufs or ext-screencopy.