yet another approach to unpremul scale pixels

This keeps punning things to premul (in anticipation of pulling
back on any unpremul support outside read/write/scalePixels),
and yet still clamps correctly for scalePixels() + unpremul + HQ
and/or gamut transform.

Change-Id: I75977cfdb94ffbe62c538ddee39f1abd2cc01935
Reviewed-on: https://skia-review.googlesource.com/106265
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index a43f95b..649c149 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -144,6 +144,9 @@
   if (skia_enable_atlas_text) {
     defines += [ "SK_SUPPORT_ATLAS_TEXT=1" ]
   }
+
+  # Temporary.
+  defines += [ "SK_LEGACY_HIGH_QUALITY_SCALING_CLAMP" ]
 }
 
 # Skia internal APIs, used by Skia itself and a few test tools.