video_capture: Implement buffer stride support for Pipewire

Add optional buffer stride support to VideoCaptureImpl and use it in
VideoCaptureModulePipeWire, allowing us to drop the requirement for
unpadded buffers. This notably improves compatibility with libcamera,
which supports a wider range of devices than the V4L2 backend - some
having hardware requirements for stride-alignment.

Extending `ConvertToI420()` in libyuv was rejected on the grounds that there are various approaches to stride handling - using a single stride value for multiple planes, having explicit values for each one etc. - as well as the fact that we might well want to extend the helper further in the future - e.g. in order to better handle color spaces such as BT709 (instead of always assuming BT601).

Thus this patch now adds a local copy of `ConvertToI420()`, supporting implicit and a single explicit stride value for now.

libyuv CL for context:
https://chromium-review.googlesource.com/c/libyuv/libyuv/+/8146821

Bug: chromium:538875176
Change-Id: Ic5781709cf5f20e8c75b9ad09d7052a355fcbec7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/491680
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Jan Grulich <grulja@gmail.com>
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48353}