The PNG reader returns a 'PNGMetadata' instance from its getImageMetadata
method. If the file being read contains bKGD, sBIT, or tRNS chunks, the
reader does not properly set the bKGD_colorType, sBIT_colorType, and
tRNS_colorType instance variables on the metadata object, thus leaving
them at a default value of 0 which signifies grayscale data.

  The "alpha" attribute of a tRNS_PaletteEntry node shows up as a negative
number if the actual value is greater than 127.

  A regression test will be placed in
test/javax/imageio/PNGMetadataColorType.java.