This is a replacement for the various GtkSourceGutterRenderers that we've had.
There have been a few downfalls with the previous gutter renderers that we
wanted to change.

First off, the gutter renderers are very sensitive to performance issues
because they are not pixel cached. I'm to blame, because I never pixel cached
those in GtkTextView years ago when adding the pixel cache. However, if we are
careful, we can generally keep our overhead lower than using the more flexible
GtkSourceView components (which need to handle more cases than we do).

Another major issue for us was that the size of the gutter was getting too
large.  The "pack renderers in" design only goes so far before you need to just
suck it up and implement a combined renderer.

This tries to coalesce as much work as we can up front so that we keep the CPU
doing similar work in chunks. That's where modern CPUs shine.

Additionally, we cache surfaces instead of pixbufs so that we can avoid a
surface creation on each render. It costs us roughly 6kb of image surface data
right now, but we can optimize that out later.

To get things packed tighter, we do some overlapping of content. By making the
debugger breakpoint arrow fill the background, we also save another 16px (plus
padding) of space.

The arrow on the breakpoint is over the line changes, so we can save another
couple pixels as well.

All in all, we have something that looks similar to before, but takes up much
less space and generally draws as fast if not faster.