Currently, the wobbly windows effect assumes that the window data will
be updated on every repaint. However, there are legit cases when the
time diff between frames can be 0, for example when per screen rendering
is on.

If we are unlucky enough and the geometry of the window changes in that
very short moment, the mapping between window quads and the bezier
patch will be wrong. The window will most likely bounce back and forth.

In order to improve handling of that tricky case, this change makes the
computeBezierPoint() function take the "uv" coordinates rather than the
absolute "xy" coordinates of window vertices. This loosens the
connection between the real geometry of the window and the cached bezier
patch, and overall makes the effect's timing code more robust.

This can be also useful if the wobbly windows effect starts accumulating
time diffs and performing the integration step every N msecs with the
purpose of maintaining...