Fix crash due to mismatch between image bounds and frame size

This crash was introduced with the change to use the vpx_img_alloc
function as it aligns both width and height.  The row change was
handled by the existing code but the new height was not.

My fix is to update the encoder code to use the d_w/d_h members which match the frame size (they are the visible area of the frame) instead
of the total area of the image.

I did this for both VPX and AV1 encoders since they both have the same
issue.

Bug: b:235325086
Change-Id: Ia5aa03e6dd6466af69ca511dc26cc029109b11a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3697469


Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1012685}