For index conversion utils, use 2d array for caching compute pipeline
state based on DrawElementsType & source offset is aligned or not,
instead of using std::map as previously.

Also moved default shader's initialization to DisplayMtl.

New test added: IndexBufferOffsetTest.DrawAtDifferentOffsetAlignments

Bug: angleproject:2634
Change-Id: I1bd77aca88e03229ef8053e32add66733e33b06e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2192569
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>