Extracted the CGContextRef creation into a virtual function of
GdkDrawableImplQuartz; implement get_context() for GdkPixmap and
GdkWindow
* gdk/quartz/gdkdrawable-quartz.c
(gdk_quartz_drawable_get_context): dropped the different
implementations; forward to the virtual function now
* gdk/quartz/gdkdrawable-quartz.h: added the virtual function
* gdk/quartz/gdkpixmap-quartz.c
(gdk_pixmap_impl_quartz_get_context),
(gdk_pixmap_impl_quartz_class_init): implemented get_context()
* gdk/quartz/gdkwindow-quartz.c
(gdk_window_impl_quartz_get_context),
(gdk_window_impl_quartz_class_init): implemented get_context()