BasiliskII: on OS X, display can slide around if initial clicks are near top of screen

[DavidLudwig]

Steps to reproduce:

1. setup a fullscreen display in the .basilisk_ii_prefs file, on an OS X host
2. launch Basilisk II
3. wait for guest OS to launch
4. move the cursor almost to the top of the screen, but not at the very top (due to Issue BasiliskII: OSX fullscreen does not recognize mouse-clicks, if cursor is at very top of screen #5, the cursor has to be slightly off from the top)
5. click and hold down the mouse's primary button
6. move the mouse downward, keeping the mouse's primary button, pressed

Expected behavior: the guest OS' menu bar does not move

Actual behavior: the guest OS' menu bar moves, in concert with the cursor's motion

Notes:

• clicking anywhere in the mid, or lower parts of the screen beforehand, prevents this bug from reproducing
• this may, potentially, be a bug in SDL itself, but I am uncertain of this.