More conservative reverse mapping bounds for SkDisplacementMapEffect

When reverse mapping bounds through SkDisplacementMapEffect, we need to
consider both the color input and the displacement input because they
could both directly, or indirectly, reference the source.

Bug: chromium:1128962
Change-Id: I03489fd2391f2ea595945961bf7e940a0e200cab
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/319656
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
2 files changed