We want to make this private, and have MetaCursorReference be
backend-defined, with the texture possibly loaded on demand.
We can't make the definition of MetaCursorReference truly private yet
because of the XFixes cursor. A victim of MetaCursorTracker trying to
do too many things at once...
I want the MetaCursorTracker to mostly be about retrieving cursor
information. Start moving the code that loads cursor images to a
new file, MetaCursor. Eventually, MetaCursorTracker's APIs will
all take MetaCursorReferences, and we can have a clean backend
split here.
