drm_hwcomposer: Split DrmDisplayCompositor::SquashAll()

Split the SquashAll function into SquashAll and SquashFrame. This
will allow us to squash arbitrary compositions without using the
active composition or applying it to the screen.

Bug: 25379136
Test: Tested on smaug

Change-Id: I1767f731e14f36540151556ce07373848b604030
Signed-off-by: Sean Paul <seanpaul@chromium.org>
diff --git a/drmdisplaycompositor.h b/drmdisplaycompositor.h
index bc19f35..975de63 100644
--- a/drmdisplaycompositor.h
+++ b/drmdisplaycompositor.h
@@ -143,6 +143,7 @@
   int ApplyPreComposite(DrmDisplayComposition *display_comp);
   int PrepareFrame(DrmDisplayComposition *display_comp);
   int CommitFrame(DrmDisplayComposition *display_comp, bool test_only);
+  int SquashFrame(DrmDisplayComposition *src, DrmDisplayComposition *dst);
   int ApplyDpms(DrmDisplayComposition *display_comp);
   int DisablePlanes(DrmDisplayComposition *display_comp);