Bug 555777 – Export to MNG animation fails

'mng_putchunk_plte' and 'mng_putchunk_trns' both copy  the array passed in
as if it was full size even when it is only partly used. This commit wraps
their calls passing the arrays dimensioned correctly.

In function 'respin_cmap', when 'find_unused_ia_colour' returns an index
suitable for transparency the number of colors in the colormap, the
following loop used to access three values beyond the end of the array
'before'.

Finally fix a typo in a call to mng_putchunk_text().