bug: handle breaks when dragging outside of window

[neftaly]

In the map handles example #491:

In Chrome:

1. Left-mouse-drag the cube outside the window
2. Hold down the right mouse button, release the left, then release the right
3. Move the cursor back into the window (with no buttons down)
4. Moving the mouse over the cube will cause it to move as if it's being dragged, even though no buttons are down

In Firefox:

1. Left-mouse-drag the cube outside the window
2. Release mouse
3. Move the cursor back into the window (with no buttons down)
4. Moving the mouse over the cube will cause it to move as if it's being dragged, even though no buttons are down

As step 2 is slightly different between Chrome and FF, I'm not 100% sure it's the same issue.

[neftaly]

also, an inconsistent behavior between chrome and ff, is that in chrome the cube continues to move when being dragged outside the window bounds, whereas in ff it stops as soon as the mouse is outside the window.