Add optimizations to the FillPathProcessor that can detect rectangles

[tocsoft]

When filling a RectangleF that doesn't overlap pixel boundries (i.e. RectangleF == Floor(RectangleF)) or filling a rectangle when anti-aliasing is disabled then we should use the faster FillProcessor rather than the vector rendering FillRegionProcessor.

FillProcessor will need updating to support passing the rectangle thru as the Source rectangle of the FillProcessor<TPixel> processor