Implement grain-merge blending mode (GIMP)
Implement grain-extract blending mode (GIMP)
Declare multiplication and division operators as const
Add a static identity transformation
Add renderer_scanline_aa_alpha
Avoid division by zero in color-burn mode

FIXME: re-work using latest math from http://www.w3.org/TR/SVGCompositing/
Avoid pixel artifacts when compositing

Change src_over alpha to avoid pixel artifacts by reordering computations.
Modify agg conv classes to allow access to the original geometry type
Avoid potential zero division resulting in nan in agg::gamma_linear
Ensure first value in the gamma table is always zero