PNG support (load, save) about grafx2 HOT 7 CLOSED

SDL_image would allow easy loading of many formats, so i'm all for going with 
that.
And yes, we'll need some code to save, but i don't think being able to save in 
jpeg is 
an interesting feature :)

Moving to 2.0 as it is quite an important feature

(using libpng, since we already have that as a dependency)

TODO:
Check memory freeing after save/load
pHYs chunk: Store and retrieve pixel ratio corresponding /wide and /tall mode
tRNS chunk: Up to 256 opacity values for the palette (entries not mentioned are 
opaque)
(Smaller filesize is achieved if the user has put all non-opaque palette entries
first. Grafx2 will not reorder the palette on its own.)

Done as of r619.
The pixel ratio is saved, it's also reloaded even if there's no automatic 
switch to
wide/tall mode.
I started the topic of Transparency as issue 110.