RenderViewport should use integer device pixels
RenderViewport should use integer device pixels

When painting a scroll offset, RenderViewport should use integer device pixels
to prevent "shimmering" effects on images.

Fixes #486

devoncarew/engine,xunmengfeng/engine,mpcomplete/engine,mpcomplete/engine,mdakin/engine,zhangxq5012/sky_engine,tvolkert/engine,abarth/sky_engine,zhangxq5012/sky_engine,TribeMedia/sky_engine,aam/engine,mxia/engine,xunmengfeng/engine,Hixie/sky_engine,mpcomplete/engine,Hixie/sky_engine,cdotstout/sky_engine,jason-simmons/flutter_engine,jamesr/sky_engine,jason-simmons/sky_engine,flutter/engine,mdakin/engine,jamesr/sky_engine,afandria/sky_engine,xunmengfeng/engine,mikejurka/engine,flutter/engine,chinmaygarde/sky_engine,jamesr/flutter_engine,jason-simmons/sky_engine,mikejurka/engine,devoncarew/engine,flutter/engine,chinmaygarde/sky_engine,iansf/sky_engine,lyceel/engine,krisgiesing/sky_engine,afandria/sky_engine,iansf/sky_engine,mikejurka/engine,mpcomplete/flutter_engine,devoncarew/engine,rmacnak-google/engine,Hixie/sky_engine,zhangxq5012/sky_engine,mdakin/engine,rmacnak-google/engine,abarth/sky_engine,qiankunshe/sky_engine,devoncarew/engine,jason-simmons/flutter_engine,afandria/sky_engine,TribeMedia/sky_engine,cdotstout/sky_engine,devoncarew/sky_engine,devoncarew/sky_engine,lyceel/engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,tvolkert/engine,aam/engine,devoncarew/engine,aam/engine,flutter/engine,krisgiesing/sky_engine,mdakin/engine,devoncarew/engine,iansf/sky_engine,abarth/sky_engine,mpcomplete/flutter_engine,chinmaygarde/flutter_engine,mdakin/engine,rmacnak-google/engine,mdakin/engine,tvolkert/engine,mikejurka/engine,devoncarew/engine,qiankunshe/sky_engine,jason-simmons/sky_engine,abarth/sky_engine,iansf/sky_engine,jamesr/sky_engine,cdotstout/sky_engine,jamesr/flutter_engine,qiankunshe/sky_engine,devoncarew/sky_engine,TribeMedia/sky_engine,zhangxq5012/sky_engine,jason-simmons/sky_engine,mxia/engine,cdotstout/sky_engine,lyceel/engine,mxia/engine,rmacnak-google/engine,chinmaygarde/flutter_engine,flutter/engine,jason-simmons/flutter_engine,jamesr/sky_engine,TribeMedia/sky_engine,jason-simmons/flutter_engine,abarth/sky_engine,TribeMedia/sky_engine,qiankunshe/sky_engine,mikejurka/engine,jason-simmons/flutter_engine,xunmengfeng/engine,mikejurka/engine,mpcomplete/engine,abarth/sky_engine,chinmaygarde/flutter_engine,mpcomplete/engine,zhangxq5012/sky_engine,devoncarew/sky_engine,lyceel/engine,krisgiesing/sky_engine,aam/engine,Hixie/sky_engine,devoncarew/sky_engine,afandria/sky_engine,aam/engine,zhangxq5012/sky_engine,mpcomplete/flutter_engine,iansf/sky_engine,jason-simmons/sky_engine,cdotstout/sky_engine,iansf/sky_engine,aam/engine,mpcomplete/flutter_engine,jamesr/flutter_engine,lyceel/engine,mdakin/engine,afandria/sky_engine,chinmaygarde/sky_engine,iansf/sky_engine,jamesr/flutter_engine,flutter/engine,krisgiesing/sky_engine,afandria/sky_engine,krisgiesing/sky_engine,jamesr/flutter_engine,tvolkert/engine,TribeMedia/sky_engine,chinmaygarde/flutter_engine,cdotstout/sky_engine,mpcomplete/flutter_engine,devoncarew/sky_engine,jason-simmons/flutter_engine,jamesr/sky_engine,mikejurka/engine,qiankunshe/sky_engine,tvolkert/engine,rmacnak-google/engine,xunmengfeng/engine,xunmengfeng/engine,zhangxq5012/sky_engine,Hixie/sky_engine,TribeMedia/sky_engine,chinmaygarde/flutter_engine,TribeMedia/sky_engine,lyceel/engine,mxia/engine,mpcomplete/engine,rmacnak-google/engine,cdotstout/sky_engine,jamesr/sky_engine,chinmaygarde/sky_engine,mxia/engine,mpcomplete/engine,jamesr/flutter_engine,jamesr/flutter_engine,Hixie/sky_engine,lyceel/engine,krisgiesing/sky_engine,flutter/engine,Hixie/sky_engine,devoncarew/sky_engine,jamesr/flutter_engine,aam/engine,tvolkert/engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,jamesr/sky_engine,rmacnak-google/engine,xunmengfeng/engine,krisgiesing/sky_engine,qiankunshe/sky_engine,mxia/engine,jason-simmons/sky_engine,qiankunshe/sky_engine,aam/engine,mpcomplete/flutter_engine,Hixie/sky_engine,mxia/engine,mikejurka/engine,mikejurka/engine,mpcomplete/engine,TribeMedia/sky_engine,jason-simmons/sky_engine,mxia/engine,iansf/sky_engine,abarth/sky_engine,chinmaygarde/sky_engine,afandria/sky_engine,chinmaygarde/sky_engine,qiankunshe/sky_engine,afandria/sky_engine,zhangxq5012/sky_engine,tvolkert/engine,jamesr/flutter_engine,mdakin/engine,zhangxq5012/sky_engine,flutter/engine,chinmaygarde/sky_engine