This include several changes:
- Color::getIndex() can return palette values with alpha != 255
- Fix Transparent and Blurs ink for indexed images to make better use
of palette entries with alpha values
- Fix bilinear resize algorithm for indexed images with alpha
- New RgbMap with four parameters: R, G, B, A
- Add one extra color scale function used in the alpha channel of the
new RgbMap
- Fix color curve, convolution matrix, invert color, and median filters
to take care of this new alpha channel on indexed images
- Fix ordered dithering and quantization
Related to #286
