JP2ECW: promote 1bit alpha band of a RGBA dataset to 8 bits to improve general user experience (can be turned off by setting GDAL_ECW_PROMOTE_1BIT_ALPHA_AS_8BIT to NO)

git-svn-id: https://svn.osgeo.org/gdal/trunk@25876 f0d54148-0727-0410-94bb-9a71ac55c965