- previously, if you supplied to_paletted and empty color map

    - previously, if you supplied to_paletted and empty color map
          and set make_colors to 'none', quant_translate() would segfault.
          This was because it was making the reasonable assumption that
          you'd have colors to map to.  quant_translate() now checks there
          is at least one color and return NULL if there isn't.
        - i_img_to_pal() now does error checking of the value returned by
          quant_translate().
        - Imager::to_paletted() now checks for success/failure of
          i_img_to_pal() correctly and does appropriate error handling.
        - i_writegif_low() did no error checking on the result of
          quant_translate(), it now does
        - we now test that trying to write a GIF image with no palette
          allowable by the quant options is a failure.
        - Imager::write() was doing nothing with the result of the call
          to i_writegif_gen(), in particular it wasn't returning () on
          failure.
        - added tests for paletted image handling and the methods
          specific to those images
        - the XS for i_setcolors() was missing the OUTPUT clause for
          RETVAL, and hence wasn't returning failure on failure.
        - supplying a sufficiently small scaling value could make the
          scale() method return an image with zero height or width.
        - the void context warning for scale() now includes the callers
          filename/line (instead of the default of Imager.pm line 15xx)
        - Imager->new will now return undef if the dimensions or number of
          channels specified for an image are out of range.  An error
          message can be retrieved with Imager->errstr.