Fix SW decompression of BC1 and add a GM to check it (take 2).

Bug: skia:9680
Change-Id: I59a4c18f7ba7d04162715cfcbeefd70f84609edb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268439
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/gn/gm.gni b/gn/gm.gni
index b360c89..fbe83d1 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -30,6 +30,7 @@
   "$_gm/backdrop.cpp",
   "$_gm/backdrop_imagefilter_croprect.cpp",
   "$_gm/badpaint.cpp",
+  "$_gm/bc1_transparency.cpp",
   "$_gm/beziereffects.cpp",
   "$_gm/beziers.cpp",
   "$_gm/bicubic.cpp",