Add blend modes: Hue, Saturation, Color, Luminosity

This should complete the list of all desirable blend modes.  Woohoo!

Also included in this commit is a super-fast RGB to HSV transform,
courtesy of http://lolengine.net/blog/2013/01/13/fast-rgb-to-hsv.  Many
thanks to "sam" for his great optimizations.

Still to-do (possibly with help from @audioglider): figure out which
transforms need bounds checking (e.g. "if r > 1 then r = 1") and which
do not.  Most the blend modes do not need these checks, which provides a
minor but welcome speed boost.  I plan to use
http://www.pegtop.net/delphi/articles/blendmodes/ as my reference for
this, but additional testing would be welcome, "just to be safe".