Fix a memory leak in CanvasImageDecoder
Fix a memory leak in CanvasImageDecoder

CanvasImageDecoder is instantiated with a callback into a Dart closure.
That closure was holding references to the ImageDecoder and the returned
Image.  The circular reference between the ImageDecoder and the callback
prevented the objects from being GCed, resulting in leakage of the image
buffer.

CanvasImageDecoder will now destroy the callback after it is invoked,
thus releasing its handle to the Dart closure.

mxia/engine,mpcomplete/flutter_engine,flutter/engine,mpcomplete/flutter_engine,jamesr/sky_engine,tvolkert/engine,mpcomplete/engine,devoncarew/sky_engine,mpcomplete/engine,jamesr/sky_engine,cdotstout/sky_engine,abarth/sky_engine,cdotstout/sky_engine,chinmaygarde/sky_engine,mikejurka/engine,jamesr/flutter_engine,Hixie/sky_engine,tvolkert/engine,mpcomplete/engine,jamesr/flutter_engine,mpcomplete/engine,Hixie/sky_engine,chinmaygarde/flutter_engine,cdotstout/sky_engine,devoncarew/sky_engine,rmacnak-google/engine,jamesr/flutter_engine,mpcomplete/flutter_engine,rmacnak-google/engine,jason-simmons/sky_engine,Hixie/sky_engine,mxia/engine,abarth/sky_engine,flutter/engine,jamesr/flutter_engine,aam/engine,mikejurka/engine,abarth/sky_engine,cdotstout/sky_engine,devoncarew/sky_engine,rmacnak-google/engine,abarth/sky_engine,devoncarew/sky_engine,Hixie/sky_engine,mxia/engine,chinmaygarde/sky_engine,mxia/engine,lyceel/engine,jason-simmons/sky_engine,krisgiesing/sky_engine,mxia/engine,flutter/engine,jason-simmons/flutter_engine,abarth/sky_engine,jamesr/flutter_engine,chinmaygarde/sky_engine,mpcomplete/engine,devoncarew/engine,krisgiesing/sky_engine,jamesr/flutter_engine,lyceel/engine,devoncarew/engine,flutter/engine,mikejurka/engine,Hixie/sky_engine,Hixie/sky_engine,lyceel/engine,chinmaygarde/flutter_engine,chinmaygarde/sky_engine,mpcomplete/flutter_engine,devoncarew/sky_engine,mikejurka/engine,chinmaygarde/flutter_engine,chinmaygarde/flutter_engine,aam/engine,jason-simmons/flutter_engine,mpcomplete/engine,tvolkert/engine,mpcomplete/flutter_engine,aam/engine,rmacnak-google/engine,flutter/engine,devoncarew/engine,cdotstout/sky_engine,mikejurka/engine,lyceel/engine,tvolkert/engine,mikejurka/engine,jason-simmons/sky_engine,jason-simmons/flutter_engine,cdotstout/sky_engine,abarth/sky_engine,krisgiesing/sky_engine,devoncarew/sky_engine,jamesr/sky_engine,jamesr/sky_engine,jason-simmons/sky_engine,chinmaygarde/sky_engine,jamesr/flutter_engine,lyceel/engine,jason-simmons/sky_engine,devoncarew/engine,chinmaygarde/flutter_engine,mikejurka/engine,devoncarew/engine,aam/engine,krisgiesing/sky_engine,jason-simmons/flutter_engine,jamesr/flutter_engine,jason-simmons/sky_engine,devoncarew/sky_engine,devoncarew/engine,krisgiesing/sky_engine,jamesr/sky_engine,krisgiesing/sky_engine,lyceel/engine,mpcomplete/engine,aam/engine,jason-simmons/sky_engine,jamesr/sky_engine,tvolkert/engine,chinmaygarde/sky_engine,aam/engine,mpcomplete/engine,mikejurka/engine,aam/engine,abarth/sky_engine,mxia/engine,lyceel/engine,jamesr/flutter_engine,jason-simmons/flutter_engine,rmacnak-google/engine,flutter/engine,rmacnak-google/engine,chinmaygarde/flutter_engine,devoncarew/engine,Hixie/sky_engine,tvolkert/engine,tvolkert/engine,mpcomplete/flutter_engine,chinmaygarde/sky_engine,jamesr/sky_engine,mikejurka/engine,mxia/engine,chinmaygarde/flutter_engine,flutter/engine,jason-simmons/flutter_engine,krisgiesing/sky_engine,flutter/engine,aam/engine,jason-simmons/flutter_engine,Hixie/sky_engine,cdotstout/sky_engine,rmacnak-google/engine,jason-simmons/flutter_engine,mxia/engine