Improve API for skipping render tasks

This lets the user query if a task is skippable, makes
the naming clearer, and only calls the virtual once.

Bug: skia:10877
Change-Id: Ia8a7e0e3014d447bd8b4e914edf4f705a2ea107b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/401856
Commit-Queue: Adlai Holler <adlai@google.com>
Auto-Submit: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
7 files changed