Make shadows pretty and configurable

The current shadow code just uses a single fixed texture (the Gaussian
blur of a rectangle with a fixed blur radius) for drawing all window
shadows. This patch adds the ability

* Implement efficient blurring of arbitrary regions by approximating
  a Gaussian blur with multiple box blurs.

* Detect when multiple windows can use the same shadow texture by
  converting their shape into a size-invariant MetaWindowShape.

* Add properties shadow-radius, shadow-x-offset, shadow-y-offset,
  shadow-opacity to allow the shadow for a window to be configured.

* Add meta_window_actor_paint() and draw the shadow directly
  from there rather than using a child actor.

* Remove TidyTextureFrame, which is no longer used

https://bugzilla.gnome.org/show_bug.cgi?id=592382