[cairo] cairo-1.13 finally opens

[Bill Spitzak]

Looking at the css and html documentation, it appears what the shadow 
does in order to combine both is:

- Compute an alpha image that is the alpha of the source intersected 
with the path

- Translate it by the "offset". This should be measured in the CTM

- There are 2 numbers that are the "spread". The CSS description of 
using a thresholded blur will not preserve antialiasing or actually 
produce the desired offset. I believe the correct rule is something like 
the following:

   - You can do this in two passes, horizontal and vertical

   - If the number is positive, find the pixel with the largest alpha 
value in a range of 2x centered on the current pixel. For non-integers 
also try the bilinear interpolation at distance x in each direction.

   - If the number is negative instead find the pixel with the lowest alpha.

   - Produce an output image of these min/max alphas.

   - IMHO if the CTM has rotation it is acceptable to perform this 
axis-aligned in either input or output space, provided the area of the 
filter is the same and it is correct for 90 degree rotations.

- Blur this alpha using a 2-pass gaussian filter. The size is specified 
in the CTM as two numbers. If there is rotation then the same 
axis-alignment as for the spread must be used (otherwise a blur smaller 
than the spread may still effect pixels on the wrong side of the path).

- Multiply this alpha image by the shadow color and composite it using 
the current operation

- Then do the rest of the drawing as normal atop this result.

I am not clear on what the various inset/outset controls being proposed 
are for. It does not look like they are relying on transforming the path 
to an inset one, which would be much more complex than the minmax 
filtering approach.

To reduce the number of entry points, could drawing the shadow be a 
separate operation? It would preserve the path. Then the current color 
can be reused to set the shadow color, and the compositing operation 
changed between the shadow and the object.