Memorize png instance in #to_png and #to_png_with_mask

Hi @meltingice,

I've just added memorisation of `@png` and `@png_with_mask` instance in `PSD::ChannelImage#to_png` and `PSD::ChannelImage#to_png_with_mask` respectively. So it can speed things up when `#save_to_png` is called multiple times on the same layer or group.

Cheers,