Summary:
A new method to tell the effects system whether the compositor scene
is able to drive animations. E.g. on software emulation (llvmpipe) it's
better to not do any animations at all.

This information can be used by effects to adjust their behavior, e.g.
PresentWindows could skip transitions or effects can use it in their
supported check to completely disable themselves.

As a first step all scripted effects are considered to be unsupported
if animations are not supported. They inherit AnimationEffect and are
all about driving animations.

The information whether animations are supported comes from the Scene.
It's implemented in the following way:
 * XRender: animations are always supported
 * QPainter: animations are never supported
 * OpenGL: animations are supported, except for software emulation

In addition - for easier testing - there is a new env variable
KWIN_EFFECTS_FORCE_ANIMATIONS to overwrite the selection.

Reviewers: #kwin, #pla...