Handle android mouse release move in Ren'Py.

Previously, we were handling this in SDL. However, this conflicted
with the MOUSEMOTION merging - the other event could be moved before
the mouse release, leading to unsightly jumps, and (perhaps) the
viewport being in the wrong place.

We now synthesize the mouse release move in Ren'Py, which lets us
control the order in which it occurs.

Fixes #391.