libcamera: colour conversion optimization

Adapted from Defy, thanks to nadlabak.
libcamera: changes to setPreviewWindow

Adapted from Crespo
libcamera: don't drop initial frames

It's not preventing garbled frames anyway.
libcamera: skip less preview frames while recording

We now use a lighter colour conversion.
libcamera: code cleanup
libcamera: request and release memory once
libcamera: crop videos recorded with the ffc
libcamera: crop preview also for third party
libcamera: fix preview heap usage