and get the shell from gimp_canvas_item_get_shell() if needed. Remove
some cairo_translate() that didn't have any effect and were leftovers
from a very early canvas item code.
not GimpCanvasLine* etc. We keep them around as items, and their
constructors return items, and it's getting more code anyway when
porting away from pause/resume, so let's keep the casting minimal at
least.
Instead, keep around the created GimpCanvasItems, and update them when
the blend coordiates change. Add setters to GipmCanvasLine and
GimpCanvasHandle which take care of calling begin_change() and
end_change() on the items around the change, so thes invalidate
properly.
This is a gross hack that typedefs around between GdkRegion and
cairo_region_t and has some evil #ifdefs. This is going to die
immeditately once we can depend on cairo 1.10.
When PROJ_ROUND()ing e.g. 3.8, it ends up at 4, then we added the 0.5
offset to draw a nice cairo line in the middle of the pixel,
effectively drawing a line that's meant to be at 3.8 at 4.5. Instead,
we now use floor(x)+0.5 now which snaps the above example to 3.5.
Also, calculate arcs like we calculate rectangles (transform the arc's
bounding box and pixel-align that, then recalculate the center), so
arcs properly align with rectangles.
