When loading, convert all >8-bit data to
GDK_MEMORY_R16G16B16A16_PREMULTIPLIED.
When saving, save all 8-bit formats as 8-bit RGBA,
and save all >8-bt formats as 16-bit RGBA.
Add support for the tiff format, which is flexible
enough to handle all our memory texture formats
without loss.
As a consequence, we are now linking against libtiff.
Using libpng instead of the lowest-common-denominator
gdk-pixbuf loader. This will allow us to load >8bit data,
and apply gamma and color correction in the future.
For now, this still just provides RGBA8 data.
As a consequence, we are now linking against libpng.
