Hide getBlendMode()

... since it mysteriously returns something even if the blender doesn't
map to any enum. Clients should use asBlendMode() or getBlendMode_or().

Change-Id: I5dc5aea51f47f297ef9b2a89535d47ac58aea9bd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/425177
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
22 files changed