Otherwise the icon "jumps" to the cursor position with its top left when
the animation starts.
This is especially visible if the dragged item is big, like when dragging
mails in Thunderbird.
We don't need the complicated wrapper system anymore,
since client-side windows are gone. This commit moves
all the vfuncs to GtkSurfaceClass, and changes the
backends to just derive their surface implementation
from GdkSurface.
We want to use a gdk_surface_new_popup for popups,
and align the constructor names with the surface
types, so rename
gdk_surface_new_popup -> gdk_surface_new_temp
gdk_surface_new_popup_full -> gdk_surface_new_popup
The temp surface type will disappear eventually.
Besides requiring it at build time, require that the server the client
is running against exposes the XInput2 protocol. We no longer fallback
on a device manager for core events.
XInput2 is more than a decade old already, and the input improvements
there (and in every other backend really) make it untenable to have
support for X11 core input events dragging things behind.
