[dawn][native] Update BufferCopy to use strong types

And updated all the code that uses it, which is a lot of code.

Also:
- Removed TypedBufferCopy as we can just use BufferCopy
- Added overload of ComputeRequiredBytesInCopy that accepts strong
  types. This overload can only be used post-validation, when there are
  no more undefined values.
- Reworked ComputeBufferImageCopyRegion overloads so that the one that
  accepts TexelCopyBufferLayout calls the one that accepts BufferCopy.
  The latter overload is more useful, and this avoids needless
  conversions from strong to weak types.
- Reworked various other bits to use strong types.

Bug: 424536624
Change-Id: If38240673ed09b2d2cc7235f07db5499ea5fcecc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/275714
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>