Cache buffers if only source crop is moved and no other changes.

 - also add trunk flag to guard it

Bug: 305718400
Test: libcompositionengine_test
Change-Id: I071d3c3610ba8516529621a0b9f56298f0439e0c
diff --git a/services/surfaceflinger/surfaceflinger_flags.aconfig b/services/surfaceflinger/surfaceflinger_flags.aconfig
index c125791..3fb763e 100644
--- a/services/surfaceflinger/surfaceflinger_flags.aconfig
+++ b/services/surfaceflinger/surfaceflinger_flags.aconfig
@@ -92,3 +92,11 @@
   bug: "299201319"
   is_fixed_read_only: true
 }
+
+flag {
+  name: "cache_if_source_crop_layer_only_moved"
+  namespace: "core_graphics"
+  description: "do not flatten layers if source crop is only moved"
+  bug: "305718400"
+  is_fixed_read_only: true
+}