cursor-tracker: Split a simple method out
cursor: Don't access the cursor tracker from the screen directly

Go through the get_for_screen getter.
cursor-tracker: Have one global cursor tracker

Now that we have a global MetaScreen, we can simply have a global
MetaCursorTracker as well. Keep the get_for_screen() API around for
compatibility, though.
cursor: Don't require an xdisplay to load cursors on the client
cursor: Always use names for cursors

Rather than sometimes using names and sometimes using glyphs.
native: Install our pointer constrainment hook after Clutter initializes

Otherwise, the evdev device manager won't be around.
plugin: Don't pass events to Clutter

We do this ourselves in the backend
backend-x11: Simplify XSync initialization handling a bit
display: Remove the RENDER extension requirement

Surprisingly, it's completely unused across all of the codebase.
Require the XSync and XShape extensions at build-time

It's been long enough. We can mandate support for these, at least
at build-time. The code doesn't actually compile without either
of these, so just consider that unsupported.