MBTILES: better detection of 4 bands dataset (if sample tile is a paletted PNG with transparency); and take into account alpha component of color table in RasterIO() (#5439)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27213 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx