Working on Follix (which uses ALS) inspired a few improvements.

v1.94 (6.6.2021)
* Added ALS_CHECK_LAYER_INDEX[].
* Added ALS_COPY_PALETTE_SEGMENT[].
* Added ALS_CREATE_LAYER_AS_CLONE[].
* Added ALS_SET_LAYER_A_AND_COLORS_FROM LAYER[].
* Made check on layer index in ALS_DESTROY_LAYER[] and ALS_MAKE_LAYER_DESCRIPTOR[] more robust.
* Removed check on layer index from ALS_GET_LAYER_DEPTH[].
* Simplified ALS_OPEN_FILE[].
* Updated manual.