- some TGA images weren't being detected correctly as TGA images

 - some TGA images weren't being detected correctly as TGA images
   https://rt.cpan.org/Ticket/Display.html?id=32925

 - handling of the left-over bit for 16-bit/pixel TGA images has been
   changed to match the behaviour of the GIMP.  Previously the bit
   being set was treated as an opaque pixel, but one user reported a
   problem with loading such an image.  I haven't been able to find any
   tools beyond the GIMP that handle alpha-channel 16-bit TGAs, so
   I'll match it's behaviour.  See issue 114913 in the GIMP's
   bugzilla.
   http://rt.cpan.org/Ticket/Display.html?id=32926