Fixes for failure to export some sprites as PNG
Detail: Buffer overflow fix for certain 24bpp + alpha sprites Ref: https://www.riscosopen.org/forum/forums/8/topics/2146?page=24#posts-121254 Attempt to write invalid output when 8bpp paletted + alpha sprite, 8bpp with per-pixel alpha is not a combination PNG allows, so this is now upscaled to 24bpp + alpha first Ref: https://www.riscosopen.org/forum/forums/5/topics/16425?page=1#posts-121264 Admin: Submission for PNG bounty.