tiff: add support for SamplesPerPixel tag in tiff_decode_tag()
Format detection and internal frame initialization is moved to a
separate init_image() function, which is called when all the tags have
been read, and so both BitsPerSample and SamplesPerPixel information
has been collected.
This fixes decoding of the file 11.tiff from roundup issue #1925.
Based on a patch by Kostya Shishkov <email@example.com>.
Signed-off-by: Diego Biurrun <firstname.lastname@example.org>