There were lots of code handling the native renderer specific cases;
move these parts to the renderer. Note that this causes the X11 case to
always generate the texture which is a waste of memory, but his
regression will be fixed in a following commit.
The lazy loading of the texture was removed because it was eventually
always loaded anyway indirectly by the renderer to calculate the
current rect.
https://bugzilla.gnome.org/show_bug.cgi?id=744932
When switching from the stage cursor to the native cursor, we
forgot to repaint the stage to get rid of the old cursor. Fix
this by having the abstract cursor renderer class track whether
we're using the backend, rather than doing chain-up shenanigans.
