cocoa_common: implement conditional video dragging

This was more roundabout than expected, since it looks like the framework
caches isMovabileByWindowBackground so in mpv's case it's needed to set it
with setMovableByWindowBackground.
cocoa_common: implement MP_MOUSE_LEAVE notifications

This is needed for the OSC. At the moment I'm sending a notification for any
mouse movement outside of the video surface. This might be overkill.
cocoa_common: fix typo in comment

Surely it's copy pasta's fault.
cocoa_common: use a tracking area to handle mouse events

This change which also flipse the coordinate system of the view, greatly
simplifies the mouse event handling code.

There are still some uglities mostly related to the cursor visibility
code. For instance the core doesn't show the cursor when it receives a mouse
leave event.
sd_ass: remove unused variable warning

The `track` variable is used only if `LIBASS_VERSION >= 0x01020000`.