move over other colortable test to visual tests
move colorizer tests to visual suite
refactor nodata related code dropping support for user-overide nodata where it does not make sense - refs #2023
followup nodata refactor - refs #2023
raster_colorizer: avoid color copy by return unsigned from get_color
fix raster colorizer via python
no need to set 'value' attribute on rasters except for point query
stub in colorize-alpha filter output to avoid crash - refs #2021
sync with 2.3.x branch