A generic "retyper" class is extracted out of the atomic counter retype
code to be used with coverting samplerCube to sampler2DArray for seamful
cubemap sampling emulation.

Bug: angleproject:3732
Change-Id: I8b5f835125b9513afcfe7baeea48afaf1299a027
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1733807
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>