[GEOT-4859]BandMerge operation should not add alpha band by default

Conflicts:
	modules/library/coverage/src/main/java/org/geotools/coverage/processing/operation/Mosaic.java
Merge pull request #530 from n-lagomarsini/GEOT-4859_11x

[GEOT-4859]BandMerge operation should not add alpha band by default (Backport on 11.x)