[AGG] 8bit AGG/PNG not transparent

[jlacroix]

When using AGG output format in 8bit, the TRANSPARENT TRUE option is not working. The background color is simply black (instead of the IMAGECOLOR yellow). I reproduce the same thing by using a predefined color palette with:

FORMATOPTION          "PALETTE_FORCE=TRUE"
FORMATOPTION          "PALETTE=palette.txt"

Here's a sample mapfile to reproduce the problem:

MAP
  NAME                      "AGG256"
  STATUS                  ON
  SIZE                    256 256
  EXTENT                  0 0 10 10
  IMAGECOLOR              255  255 0
  TRANSPARENT             TRUE
  IMAGETYPE "AGG"
  OUTPUTFORMAT
    NAME                  "AGG"
    DRIVER                AGG/PNG
    IMAGEMODE             RGB
    FORMATOPTION "QUANTIZE_FORCE=ON"
    FORMATOPTION "QUANTIZE_DITHER=OFF"
    FORMATOPTION "QUANTIZE_COLORS=256"
  END
 
  LAYER
    NAME                  "line"
    STATUS                DEFAULT
    TYPE                  LINE
    CLASS
      NAME "Province"
      COLOR 255 0 0
    END
    FEATURE
      POINTS
        2 2 5 5 5 1 9 9 
      END
    END
  END
 
END