[d3d12] Support padded `rows_per_image` for 3D texture writes

[teoxoy]

See #3992 (comment).

Impact of this not working is limited but as a user I imagine it would be tough figuring out why things don't behave as expected.

[jimblandy]

Promoting to P1 because it's a mess it looks hard and risky.

[jimblandy]

This seems to have been discussed by the committee here:
gpuweb/gpuweb#3185

[ErichDonGubler]

@cwfitzgerald: Were you going to push this over the line, or should Firefox re-assign?

[cwfitzgerald]

I'm not.