Dry up fixture comparison in embedder unit-tests. (#11359)
Dry up fixture comparison in embedder unit-tests. (#11359)

No functional change. Just makes testing with fixture images easier. Adding a
whole lot more tests that use this path for the embedder surface rotation
patches. Want to land stuff in smaller chunks.
jamesr/flutter_engine,cdotstout/sky_engine,jamesr/flutter_engine,jamesr/flutter_engine,devoncarew/engine,tvolkert/engine,jason-simmons/sky_engine,cdotstout/sky_engine,jamesr/sky_engine,chinmaygarde/flutter_engine,flutter/engine,jason-simmons/flutter_engine,chinmaygarde/sky_engine,mikejurka/engine,Hixie/sky_engine,Hixie/sky_engine,mikejurka/engine,devoncarew/engine,rmacnak-google/engine,rmacnak-google/engine,jamesr/flutter_engine,chinmaygarde/sky_engine,chinmaygarde/sky_engine,jamesr/sky_engine,tvolkert/engine,jason-simmons/sky_engine,devoncarew/engine,chinmaygarde/sky_engine,jamesr/flutter_engine,jason-simmons/flutter_engine,cdotstout/sky_engine,mikejurka/engine,tvolkert/engine,jason-simmons/flutter_engine,jamesr/flutter_engine,jamesr/flutter_engine,devoncarew/engine,tvolkert/engine,aam/engine,chinmaygarde/flutter_engine,devoncarew/sky_engine,flutter/engine,jason-simmons/sky_engine,mikejurka/engine,cdotstout/sky_engine,devoncarew/sky_engine,flutter/engine,rmacnak-google/engine,mikejurka/engine,tvolkert/engine,rmacnak-google/engine,Hixie/sky_engine,tvolkert/engine,tvolkert/engine,cdotstout/sky_engine,jason-simmons/flutter_engine,Hixie/sky_engine,rmacnak-google/engine,jason-simmons/sky_engine,jamesr/flutter_engine,flutter/engine,mikejurka/engine,jamesr/sky_engine,chinmaygarde/flutter_engine,devoncarew/sky_engine,chinmaygarde/flutter_engine,aam/engine,jamesr/sky_engine,chinmaygarde/sky_engine,chinmaygarde/flutter_engine,rmacnak-google/engine,Hixie/sky_engine,devoncarew/engine,rmacnak-google/engine,devoncarew/sky_engine,aam/engine,devoncarew/engine,jason-simmons/flutter_engine,jason-simmons/flutter_engine,mikejurka/engine,devoncarew/engine,jason-simmons/flutter_engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,mikejurka/engine,jason-simmons/sky_engine,flutter/engine,jamesr/flutter_engine,aam/engine,chinmaygarde/sky_engine,jamesr/sky_engine,jason-simmons/sky_engine,mikejurka/engine,flutter/engine,devoncarew/sky_engine,devoncarew/sky_engine,flutter/engine,jamesr/sky_engine,aam/engine,jason-simmons/sky_engine,cdotstout/sky_engine,aam/engine,aam/engine,Hixie/sky_engine,flutter/engine,devoncarew/sky_engine,aam/engine,chinmaygarde/flutter_engine,jamesr/sky_engine,Hixie/sky_engine,Hixie/sky_engine,chinmaygarde/sky_engine,cdotstout/sky_engine