Make fallback descriptors explicit

Have the subruns carry a pointer to the descriptor they need.
This simplifies the regen code, and makes ARGB fallback more explicit.

Change-Id: If11477cb9ecccc90ae262327e0579f4ab249836e
Reviewed-on: https://skia-review.googlesource.com/c/171586
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Herb Derby <herb@google.com>
4 files changed