nd-window: Add capsfilter for virtual display sources

For virtual displays, add a capsfilter named scrfilter in between the pipewiresrc and the intervideosink. With this filter, the source canvas resolution provided by mutter is re-negotiated to the desired stream output resolution (currently hardcoded to 1920x1080@30) instead of being lossily scaled to the desired resolution from the default virtual display resolution hardcoded in mutter (1280x780@60). To avoid confusion with the new capsfilter, the capsfilter previously named sizefilter is renamed to sinkfilter.

Fixes #352 (closed)