Minimal changes to the code to allow using bigger bitmaps from
wxBitmapBundle in high DPI instead of having to create individual
bitmaps with scale factor greater than 1.
Notice that this is not actually used anywhere in wxGTK yet, but will be
soon.
Document which pointers can, and can't, be null and when exactly is the
stored bitmap valid, as this is far from being immediately obvious.
No real changes.
And use it to get HiDPI support wherever we use GtkImage. This extends and
consolidates support for custom drawing of images which has already been added
somewhat redundantly in several places.
