effects/wobblywindows: Allow model geometry and real geometry get out of sync
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 uniform "wobbliness" across different refresh rates.