Support for multiple input transparent colors for grid formats

GeoTools currently support only one color as an input transparent color. For some use cases it is not enough as some raster datasets require multiple colors to be rendered as transparent. The patch adds suport for specifying multiple transparent colors.
A new ParameterDescriptor (INPUT_TRANSPARENT_COLORS) was added in class org.geotools.coverage.grid.io.AbstractGridFormat.
GeoTiff and ImageMosaic readers were extended to properly handle this parameter.