Make ScreenCapturerSck crop frames, reconfigure on source size changes, and other improvements

To prepare for handling other sources than just screens, e.g.
single-window, multi-window, and any source with presenter-overlay, this
commit mainly does:
- crops incoming frames to the bounding rect
- reconfigure the capturer on the first CaptureFrame after receiving a
  frame where the source had to be scaled down because the configured
  size was too small
- avoid handling an incoming frame if there was no change to its content

Bug: webrtc:367915807
Change-Id: Ia5e1b3fe82628eadc8d6ec96230ce2ca8e98c603
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/365092
Reviewed-by: Johannes Kron <kron@webrtc.org>
Commit-Queue: Andreas Pehrson <apehrson@mozilla.com>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/main@{#44354}