Add support for ILBM "Hold and Modify" mode images
This patch attempts to add support for importing ILBM images with "Hold and Modify" mode set.
In this mode, the last two bits in a plane are used to determine if a palette color should be used - or if the prior pixel value should be modified to create a new color (by replacing either the red, green, or blue channel with the plane bits).
This supports HAM 5/6/7/8 modes.
Edited by Alx Sa