Add SK_SUPPORT_LEGACY_BITMAP_COPYTO flag

TBR=reed@google.com
NO_TRY=true

BUG=skia:6464

Review-Url: https://codereview.chromium.org/2844223003
Cr-Commit-Position: refs/heads/master@{#467678}

skia/config/SkUserConfig.h

@@ -206,6 +206,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #   define SK_IGNORE_GPU_DITHER
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_BITMAP_COPYTO
+#define SK_SUPPORT_LEGACY_BITMAP_COPYTO
+#endif
+
 // Remove this after we fixed all the issues related to the new SDF algorithm
 // (https://codereview.chromium.org/1643143002)
 #ifndef    SK_USE_LEGACY_DISTANCE_FIELDS