The WebKit canvas code assumes that antialiasing and average image quality is enabled by default. The Qt ImageBuffer implementation does not however set any default render-hints causing the defaults in the GraphicsContext to be antialiasing and smooth scaling disabled.
Ideally these default should follow the render-hints set on the webview, but I don't see how that is possible currently, so let us default the defaults to the default.
Btw, this is Issue 2 from https://bugreports.qt-project.org/browse/QTBUG-33081.
Created attachment 210360 [details]
Created attachment 210372 [details]
Moved the fix to better defining image-interpolation quality in TextureMapper since that is where the problem is.
Committed r154985: <http://trac.webkit.org/changeset/154985>