avdevice/x11grab: fix cursor drawing in multi-screen setup

The code uses XFixes to retrieve the cursor coordinates, but XFixes
gives no information of what screen the pointer is on; this results in
always drawing the cursor on the captured screen even if the mouse
pointer was on another screen.

For example, when capturing from screen 1 (i.e. -f x11grab -i ":0.1")
the cursor was being drawn in the captured image even when the mouse
pointer was actually on screen 0, which is wrong and visually confusing.

Use XQueryPointer to check that the pointer is actually on the screen
which is being captured.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
