Added posterize filter
Added posterize filter

Posterize reduces the number of colors. Lower values for "step" results in fewer colors.

mcanthony/texgen.js,sojimaxi/texgen.js,mcanthony/texgen.js,sojimaxi/texgen.js,mrdoob/texgen.js,mcanthony/texgen.js,mrdoob/texgen.js,mrdoob/texgen.js,sojimaxi/texgen.js