fix: Handle window drag by touch input

[aleasto]

global.get_pointer() does not track touch input, so when initiating a drag via touchscreen we would be incorrectly tracking the mouse position instead.

Unfortunately, in GNOME 50 there is no API to get information on the touch point that is driving the window drag from Display::grab-op-begin. The best we can do is track global events happening before grab-op-begin and try to infer which touch point is driving the drag. Specifically, the global stage receives a LEAVE event from the device starting the window drag immediatly prior to Display::grab-op-begin.

For this implementation we use the Clutter.Sprite API which is new in GNOME 49, and specifically Cluter.Sprite::get_coords() which is new in GNOME 50, but we fall-back to global.get_pointer() to support older shell versions, where however the bug will not be fixed.

[vanvugt]

Unfortunately, in GNOME 50 there is no API to get information on the touch point that is driving the window drag

There might be an existing API hiding under the name of "cursor" or "sprite" as it's now called. Just get the current one relevant to the drag. Oh you already found that.

[aleasto]

Yes but there's not enough information in the Display::grab-op-begin callback to query the right Sprite, so we have to go through other signals first.

[Leleat]

I don't know enough about GNOME Shell's/mutter's internals to really review this (nor do I have a touch device to test this). But since this doesn't seem to interfere with the existing behavior, I'll merge it.

Could you just please "disconnect" the global.stage signal in the destroy method?