[matplotlib-devel] cairo patch #4: Fix draw_arc

The draw_arc code was putting all arcs centered on the origin instead
of the proper location. This patch fixes that.

With this, plus my earlier patch #2 to enable clipping, the
line_styles.py example is now rendering quite well with the cairo
backend.

However my patch #1 to add the snapping is messing up line_styles in
some places. For example, where there is a strokes sine wave the
snapping is interfering with its proper shape.

Obviously, snapping smooth, curved user data being plotted like that
is a really bad idea. Things that should be snapped are things like
frames, grid lines, ticks, and object borders, particularly when
aligned with an axis.

-Carl