Please describe the problem:
I installed (from source) babl-0.0.22, gegl-0.0.18 and gimp-2.5.1.

From ufraw I have produced a PNG-file, and this is the info pngtool gives:

dsc_0118.png...
  Image Width: 2014 Image Length: 1712
  Bitdepth (Bits/Sample): 16
  Channels (Samples/Pixel): 3
  Pixel depth (Pixel Depth): 48
  Colour Type (Photometric Interpretation): RGB 
  Image filter: Single row per byte filter 
  Interlacing: No interlacing 
  Compression Scheme: Deflate method 8, 32k window
  Resolution: 0, 0 (unit unknown)
  FillOrder: msb-to-lsb
  Byte Order: Network (Big Endian)
  Number of text strings: 3 of 9
    Software (tEXt uncompressed): UFRaw
    Source (tEXt uncompressed): NIKOND40
    Raw profile type exif (tEXt uncompressed): [.. cut ..]

I open the file in Gimp, and without doing anything else, I save it with a new filename: gimp.png

This is what pngtool states about this new file:

gimp.png...
  Image Width: 2014 Image Length: 1712
  Bitdepth (Bits/Sample): 8
  Channels (Samples/Pixel): 3
  Pixel depth (Pixel Depth): 24
  Colour Type (Photometric Interpretation): RGB 
  Image filter: Single row per byte filter 
  Interlacing: No interlacing 
  Compression Scheme: Deflate method 8, 32k window
  Resolution: 2835, 2835 (pixels per meter)
  FillOrder: msb-to-lsb
  Byte Order: Network (Big Endian)
  Number of text strings: 0 of 0
  Offsets: 0, 0

As you see, both bitdepth and pixel depth are reduced. In addition the filesize has been reduced to almost 50%. Does not this mean that data in the image is lost?

Steps to reproduce:
1. Open the png-file in GIMP
2. Save the png-file in GIMP
3. 


Actual results:
Both bitdepth and pixel depth are reduced.

Expected results:
The file should be 100% similar

Does this happen every time?
Yes

Other information:
In addtition, the exif-information is lost, even I compiles Gimp with libexif-support. But this is covered in bug #56443