[skotty] Initial opacity support

TBR=

Change-Id: I62581d3a7a83af5ccf373f0f4edf66a2d7f06f07
Reviewed-on: https://skia-review.googlesource.com/92223
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 0dee148..1ca9b44 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1384,6 +1384,7 @@
       "experimental/sksg/SkSGNode.cpp",
       "experimental/sksg/SkSGPaintNode.cpp",
       "experimental/sksg/SkSGRenderNode.cpp",
+      "experimental/sksg/effects/SkSGOpacityEffect.cpp",
       "experimental/sksg/effects/SkSGTransform.cpp",
       "experimental/sksg/geometry/SkSGMerge.cpp",
       "experimental/sksg/geometry/SkSGPath.cpp",