Objects of this class are used as keys to a hash map and contain 10
serials.  Prior to this change, operator== would compare all 10 serials.
This change tracks the maximum index set and only compares up to that
many serials.

To make sure this maximum index stays low in the presence of
depth/stencil attachment, this class stores the depth/stencil
attachment's serial before the color attachments.  The update API of the
class is improved so this implementation detail is not leaked.

This is in preparation for supporting resolve attachments, which would
add another 8 serials to the class.  With this change, the performance
of cases that don't use resolve attachments won't be affected.

Bug: angleproject:4836
Change-Id: Ia4874ab79163901fb86f5bee4120e9f19babdc09
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2306514
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>