pdFloodFill: add antialiasing support

For contiguous fills, a custom AA solution is used.  Since we know which
pixels were checked but not filled, we use that as our guide for which
pixels need to be antialiased.  This makes the function extremely fast,
as we only process the smallest possible number of pixels.

For global fills, PD's standard quickBlur function is used at a radius
of 1px (since we don't have a continuity map available).  This means
GDI+ is used for the blur on Win 7 and later, while a standard box blur
is used on XP or Vista.