Using boxed types, this change allows the compiler to catch errors when
a level index in one space (e.g. GL) is mistakenly used in another space
(e.g. VK).

This change uncovered a number of bugs due to such mistakes which are
fixed.

Mistakes are still possible when the index is explicitly extracted, for
example to be given to a Vulkan command, or when it's created, for
example when retrieved from gl::ImageIndex::getLevelIndex.  Future work
can include using gl::LevelIndex in gl::ImageIndex directly to alleviate
the latter at least.

Bug: angleproject:4880
Change-Id: I6427c68c3bc096f771402f51c8554d8171758aa9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2380232
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>