* Rename ld_canvas_translate_*_coordinates() to ld_canvas_*_to_*_coords().
* Add missing parameter assertions to those functions.
* Rename local variables in draw_grid().
* Use gdk_cairo_rectangle() in expose-event handler.
* Update comments.
1. Add methods for coordinate translation between cairo/GtkWidget
coordinates and LdDocument coordinates.
2. Draw the grid correctly (also make it significantly faster).
3. Place the canvas into a GtkScrolledWindow and register
the set_scroll_adjustments signal in the GtkWidget base class.
4. Allow scrolling inside of an area of 200 x 200 units (for now).
In the future, it should be possible to go to infinity.