[ePicLoad] Improve fill colour matching for colourmapped images

[prl]

In ePicLoad::getData() the background fill colour is colour matched to the nearest colourmap entry for the image, and that colour is used for the background fill. That colour may be quite different from the background fill colour specified in ePicLoad::setPara(). In particular, if there are no transparent colours in the image's colourmap, the fill cannot be transparent.

A suggested enhancement is to add the exact background colour to the colourmap if it isn't already in the colourmap, and the colourmap has some unused entries.

[Peter Urbanec]

Fix bug #546: [ePicLoad] Improve fill colour matching for colourmapped images

If there is no exact match for the ePicLoad::setPara() background colour in the image's colourmap, add that colour to the colourmap so that it can be used for the background, provided there are slots available in the colourmap.

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