Nice looking scaledown with mipmap emulation

Add MutterTextureTower, an abstraction for getting a image with
the right level of detail for rendering at a particular scale,
by manually scaling down by powers of two.

This results in much better looking scaled window images when
mipmaps can't be used with texture_from_pixmap (which is the
typical case for current GL drivers.)

When framebuffer objects are available, they are used to do
the scaledown using the GPU without having to pull the data
back from video memory. A software codepath is also available
for the case when FBO's are not present, though performance
will suffer

https://bugzilla.gnome.org/show_bug.cgi?id=601032