Implement ext-image-capture-source and ext-image-copy-capture

[Ottatop]

This PR implements ext-image-capture-source and ext-image-copy-capture. Most importantly, this enables window capture.

The git version of xdg-desktop-portal-wlr is recommended for pipewire window capture until they make a new release.

Todos:

• Cursor sessions
• Dedup with the wlr-screencopy impl somewhat Ok lowkey I'm looking to remove this sometime in the future so I'm not gonna bother
• Check xwayland window capture
• Fix damage when the output is scaled
• Check if it works when the output is transformed
• Probably don't render decorations when capturing windows
• Make window capture work when the window isn't visible
  • Given that this doesn't work in sway I'm pushing this off
• Investigate optimizations for output capture
  • Update: let's not overcomplicate this for now
  • The current wlr-screencopy impl works by copying the results of the next frame that will be displayed on an output. This requires minimal overhead. However, the current impl of copy-capture renders new frames from scratch for each active capture session, similar to what Niri does. I believe this is required for toplevel capture, but it may be worthwhile to port the copying parts of the wlr-screencopy impl when doing output capture. Or roll that in when deduping.
• Left an empty file in there
• Stop sessions when output/window disappears
• Final cleanup and perf testing
• Probably a bunch of other stuff