[Qt] Images scaled poorly on composited canvas

[Qt] Images scaled poorly on composited canvas

https://bugs.webkit.org/show_bug.cgi?id=120632

Reviewed by Jocelyn Turcotte.

Explicitly set a imageInterpolationQuality on the TextureMapper, because
InterpolationDefault may be interpreted differently by nested GraphicsContexts.

* WebCoreSupport/TextureMapperLayerClientQt.cpp:
(TextureMapperLayerClientQt::renderCompositedLayers):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154985 268f45cc-cd09-0410-ab3c-d52691b4dbfc

Task-number: QTBUG-33081
Change-Id: Iec90f53da38c4520cef2aac0519e36af74dd5a50
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>