PCX: support for more formats
Added support for:
- (R/O) 1 bit, 3 planes (ICONDOC.pcx) -> test case from internet archive
- (R/O) 2 bits, 1 planes (CGA_FSD.pcx) -> test case from internet archive
- (R/W) 8 bits, 4 planes (dice_rgba.pcx) -> supported by ImageMagick and GIMP 3
Fix write support for:
- Grayscale 8/16-bits (saved as indexed)
- Alpha 8 (saved as indexed)
- RGB 16-bits depth (saved as RGB 24/32)
Edited by Mirco Miranda