[cairo] OVER / SOURCE optimization for cairo_paint

[Antoine Azar]

Hi Behdad,

thanks for the feedback.

>cairo.c should really be a wrapper around gstate.  And gstate should 
>really be a wrapper around surface and scaled-font.  This particular
>optimization for example is more suitable to go in the surface 
>layer, for example because we use cairo_surface_paint() internally when we
>don't have and don't need any gstate.

I see. I'll look into that, but I don't want to clutter an existing 
layer of code with various optimizations. Don't you think it would be 
nicer to add a "dispatching" layer somewhere below cairo.c and above 
cairo-surface.c that would be responsible for such optimizations? Is 
there any design doc about our current code paths?

> > +                    surface_rect.x = floor(surface_rect.x);
> > +                    surface_rect.y = floor(surface_rect.y);
> > +                    surface_rect.width = ceil(surface_rect.width);
> > +                    surface_rect.height = ceil(surface_rect.height);
>
>Why is the floor/ceil needed here in the first place?

That's actually a good question (which I ask in a comment just below 
:) ). If you look at the filter_nearest_offset test, it checks 
exactly that. It increments the source's matrix translation by 0.1 in 
a for() loop, and expects a change in output only when we reach an 
integer increment. I don't understand why. Without flooring and 
ceiling, the test fails.

I'll fix the other issues you pointed out.
Thanks,
Antoine