MacOS: Don't remove pointer after drag if released inside window

[knopp]

Fixes flutter/flutter#83494

This PR adds NSTrackingEnabledDuringMouseDrag flag to tracking area options, which causes cocoa to signal mouseEntered: when mouse cursor enters the window during dragging. In mouseEntered: the has_pending_exit is then cleared, so flutter no longer removes the pointer if mouse is released after cursor returns to window.

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[stuartmorgan]

Interesting, I didn't know about this. My last deep wrestling with complex mouse tracking predated 10.5 IIRC :)

[knopp]

Yeah. The default behavior where you get mouse exit but not enter during the same drag is just weird.

[knopp]

@stuartmorgan, I updated the commend and add the guard in mouseEntered:

[stuartmorgan]

LGTM