Instead of writing the bitset masks, iterate up until the last active
buffer. Write zeros instead of skipping spaces. This is a bit simpler
to implement and also fixes a bug where empty buffers could cause us
to write invalid handles.

Bug: angleproject:5736
Change-Id: I785ef18ef5ae45109ec7d6e0b079b79a9984a1f8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2837848
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>