Cache scaled surfaces

Create a utility class called CachedSurface. This maintains a scaled version of a pixbuf so that if it is redrawn at the same size, it does not need to be scaled again.

Use this class in the GalleryPage and GalleryButton components. According to some simple tests, this can save time on the order of several milliseconds per draw.

Also, took the opportunity to simplify the drawing code by removing matrices and other weird stuff that I don't remember why I put it there.

Fixes #13 (closed).