Define SK_SUPPORT_LEGACY_ALPHA_BITMAP_AS_COVERAGE

This flag, along with I01fadaed52eecc42416deb2307d03499bca85a4c (in
upstream Skia), restores the old behavior of treating alpha images as
coverage in the CPU backend. This fixes a regression in a technique apps
use to apply rounded corners to images.

Bug: 231400686
Test: GM in Skia
Change-Id: I4857da2c3dd54e64ca86eb1ea2122096625f8662
(cherry picked from commit 66e866e7f699d0bf3f7c0adde896fdbab7ecab67)
Merged-In: I4857da2c3dd54e64ca86eb1ea2122096625f8662
1 file changed