PNG files: add handling for tRNS chunk in 24bpp images

This is a rare case where GDI+ handles the image correctly, but
FreeImage does not.  24bpp PNG files can contain a single transparent
color, as specified in the tRNS chunk.  GDI+ will automatically upscale
an incoming 24bpp PNG with these conditions to 32bpp, and use the tRNS
chunk to generate an alpha channel.  FreeImage will not.

Thanks to the magic of ExifTool, we can now catch this case, and apply
the transparency manually.
PNG files: remove special handling for tRNS chunk in 8bpp images

FreeImage handles these correctly, so no need to do our own custom
handling.  (Only 24bpp images cause trouble.)