Wayland capture: validate buffer geometry before pixel copy

Reject Memfd buffers where stride exceeds the mapped region to prevent
out-of-bounds reads. Cap stream dimensions to prevent integer overflow
in frame allocation. Bound cursor bitmap stride to reject oversized
values.

Bug: chromium:509294495
Change-Id: I524365383d4d68896377b1fe95e465c56e22a699
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/469682
Reviewed-by: Andreas Pehrson <apehrson@mozilla.com>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Jan Grulich <grulja@gmail.com>
Cr-Commit-Position: refs/heads/main@{#47627}