re-use duplicate colors

[ChrisBarker-NOAA]

The user may want to use multiple names for the same color. It would be nice if the code could check for this, and re-use the color index, simply adding the new name.

This would require a two way mapping -- name to color and color to name.
(and mapping to the index, too)

[ChrisBarker-NOAA]

note: this should probably use:

gdImageColorExact

rather than trying to keep track in the cython code.