kwineffects: Introduce DeformEffect

DeformEffect is the base class for effects that transform the window
quad grid, e.g. wobbly windows or magic lamp. The main difference
between normal effects and offscreen effects is that the latter renders
animated windows into offscreen textures which are later deformed.

The offscreen texture approach is superior to the half-pixel correction
approach as it produces more visually appealing results. Even with
half-pixel correction, you're still going to see jagged lines between
the main surface and the server-side decoration or drop-shadow.

It is also needed to reduce the number of usages of WindowPaintData::quads
which is needed to move forward with the scene redesign goal.