-GUI update and string changes.
-Closes enhancement ticket 2024
-Accomplished by caching the last QgsRasterViewPort, I see this as a bit of a work around rather than a real solution. The issue at hand is that the render context can only be provided by the map canvas so neither the layer itself nor an layer properties dialog has access to the render context.
git-svn-id: http://svn.osgeo.org/qgis/trunk@12015 c8812cc2-4d05-0410-92ff-de0c093fc19c