Color problems when using raster symbols and 8 bits PNG

[dmorissette]

We have noticed a problem with MapServer V4.0 when drawing raster symbols on a
map and using 8 bits PNG output.  Sometimes the color of the raster symbol will
change.

In our specific case, we were using a png symbol that contained red color.  
- If the rest of the map also contained some red elsewhere (in the roads layer
for instance) then the symbol was displayed in red correctly
- If there was no red color anywhere else in the map then the symbol was drawn
in black instead of red.

The above was happening when using 8 bits PNG output.  Switching to 24 bits PNG
or JPEG output resolves the issue.  Sounds very much like a color table issue.

Please keep in mind that this happens with raster symbols, not vector symbols.

Unfortunately we don't have a simple reproduceable test case for this.  If we
can build one then we'll attach it.