Backport of latest fixes to KF5
- PCX: fix wrong channel order with some image formats (from MR !150 (merged))
- PCX: write/read right resolution (from MR !150 (merged))
- PCX: Save images with depth greater than 24-bits (from MR !150 (merged))
- PCX: Fix reading of the extended palette (from MR !129 (merged))
- TGA: fix alpha detection and speed improvements (from MR !151 (merged))
- SGI: fix alpha detection and image size limit overflow (from MR !151 (merged))