Re-enable colorize-alpha for 2.2.0

colorize-alpha was removed 97fed08ef5aed29fc82620bddd578da5389e4ee5 in but is available in 2.2.0

https://github.com/mapnik/mapnik/blob/2.2.x/include/mapnik/image_filter_types.hpp#L128
Remove stray filters from 2.2.0

The rest of these filter entries were removed in 97fed08ef5aed29fc82620bddd578da5389e4ee5
Merge pull request #57 from gravitystorm/colorize-alpha

Fixes for colorize-alpha in 2.2.0