moved png_set_add_alpha to before png_read_update_info

On my system, you need to call png_read_update_info after
png_set_add_alpha or block textures without an alpha channel will fail
to load with "sequential row overflow".

Also I added a missing delete[].
Merge pull request #11 from agrif/pngfix

moved png_set_add_alpha to before png_read_update_info