static fun combinationExists(dir: java.io.File, filename: java.lang.String, cleanupIfNot: boolean): boolean
Check if the RGB and ALPHA channel's exists
dir - java.io.File: saveImage directory
filename - java.lang.String: the filename prefix
cleanupIfNot - boolean: delete the second channel if the other not present
Return
boolean: true, if the RGB and ALPHA channel is present.