fix #9984 - Add support for WMS tiles defining a transparent color in RGB space (tRNS PNG chunk for example), instead of a proper alpha channel. Surprisingly, Java does not support that out of the box, ImageIO.read always returns opaque images. Allows to switch between this mode and standard mode using WMS layer contextual entry "Use Alpha Channel", for consistency with images defining an alpha channel. Does not impact other images than WMS tiles.