[glsl] Use structs for padding when possible

When the number of padding bytes is large and the current offset is
aligned, use structures to pad out the struct to avoid emitting too
many members and initializer values. We use structures of uint values
to avoid increasing the alignment of the containing struct.

This significantly reduces the number of members needed for large
amounts of padding, which helps with some fuzzer cases.

Also add proper handling for padding amounts smaller than 4 bytes,
which can happen with f16 members.

Change-Id: I8ccb22a64ed32a1bec92ee2e4b12c945e8409cfa
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/336055
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: James Price <jrprice@google.com>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>