Added an Image::GetAsSourceSurface to return a gfx::SourceSurface
and implemented in terms of gfxPlatform::GetSourceSurfaceForSurface.
Deprecated GetAsSurface.
(Detail: Chose to put an #include for Point.h in a header instead of a
forward declaration because a fwd decl caused an "Already defined" error
and having neither fwd decl or #include caused another compile failure
saying IntSize was undefined)