remove contextscalewrapper.h header from osx includes
[[ RefactorGraphics ]] Various fixes to libgraphics

add MCGRectangleTranslate function
make MCGRectangleIntersection public - fix so doesn't return negative width / height
add MCGImageCreateWithRasterAndRelease
change copy param of MCGRasterToSkBitmap to ownership type (take, borrow, or copy)
[[ RefactorGraphics ]] factor together functions that create a CGImageRef from a bitmap
add Composite() method to MCStackSurface
implement MCMacStackSurface::Composite(…) on OSX