Improve dock window dragging.

[tom-seddon]

Two minor tweaks:

• cancel drag if the mouse position becomes invalid. Let the window float, same as when the left button is released
• when making a window float, clamp the window position. The new size might be smaller

(I was hitting the second case after docking a small floating window into a large dock area. It's then much larger when dragged out of its dock than it will be when it starts floating - so it's possible to move it somewhere that'll leave it completely off the screen when it switches into floating mode.)

Thanks,

--Tom

[Flix01]

Merged. Thank you!