[Roadster] new gdk renderer and drag to scroll

[Ian McIntosh]

I added a GDK renderer to Roadster as an alternative to Cairo.  It's
about 50 times faster than Cairo.  Carl, you've got some work to
do... ;)

It's super easy to switch between GDK and Cairo, and Roadster can even
use each one to draw different elements of one image.

Once Cairo is fast enough to render a full-screen frame in ~0.5 seconds
(it's at maybe 6 seconds now), or where hardware acceleration is
present, we can use GDK while animating and Cairo on the final frame.
(Or Cairo for everything, if fast enough.)

Since drawing is fast now, I added drag-to-scroll functionality (no
threading needed!) and the mouse scrollwheel zooms in and out.

Currently, text rendering (which always uses Cairo) is still pretty
slow, but I hear from Carl that something must be broken and it will get
much faster.  Text rendering is disabled while dragging.

The database is the new bottleneck.  Now it seems crazy that we're
loading from disk for EVERY FRAME drawn (while dragging!!).  Some kind
of memory cache may be the next step.

-Ian