I personally feel now, after months of thinking about it, that AGS should move on from Allegro 4. However, that doesn't mean move away from Allegro all together. But I am open to other libraries as well. The big thing for me is portability. Pretty much all these libraries support Mac, Windows and Linux. Great. But I also own/have owned portable devices such as the PSP and Android (No longer have the psp though as I sold it) and my brother has an IPhone and IPad. We do most of our gaming on these devices rather than on our pc's. So to me, its a big deal to have those ports. It seems as if, even though its possible to port, SDL and SFML do not have official ports to those systems. No biggie I Suppose. With extra hard work, it can be done. But it makes things challenging. Here are my observations:
Latest updates:
SDL was last updated in January 2012
SFML was last updated in April 2012
Allegro was last updated in January 2013.
Ports:
SDL will function officially with Windows/Win CE, MacOS, BSD, Linux, Solaris, Irix and QNX along with unofficial ports to OS that people generally do not use anymore.
Allegro officially supports Windows, Mac, Linux, IOS, Android. Older Versions support PSP, QNX, BeOS, etc... To me, you can probably easily port version 5 to those OS as well.
SFML officially supports Windows, Linux and Mac with a promise to support more in the future.
Ease of use:
SFML - Have not messed around with it, so I couldn't tell you.
Allegro - To me, as a sort of novice C++, Allegro is complicated to me.
SDL - Even to me, this is fairly easy to implement.
Convenience:
AGS already uses Allegro. So I would think that moving from 4 to 5 would be a somewhat smooth transition. On the other hand, since SDL is easier to understand, you would have also have more of the developers understanding how to use it and improve the engine. But again, I don't think it would be that much harder with Allegro being as the developers seem to be doing very well with it.
Mixing with other libraries:
Pretty much all of them support mixing with OpenGL or DX. However, my understanding is that Allegro is far more modular than SDL or SFML, allowing you to really integrate seemlessly with other libraries. So for instance, if you wanted to use OpenGL ES for portable versions of AGS, or if you wanted to include 3D capabilities with Ogre3D, Allegro can do that easily and without any mixed interference.
Community:
SDL seems to have diminished as a community and from what I hear, support from the development team is non-existent. I haven't really checked what its like with Allegro, but it might be the same, it might night. SFML Seems to be somewhat still active on the forums though.
Speed of updates:
In order for AGS to continue to have timely updates, it should stay with what is familiar. If AGS was to move on to SDL or SFML, it would probably require a whole new rewrite of much of the engine, thus probably taking a year or longer to achieve.
Here is a good overview of these libraries that can really tell us which is better for AGS:
http://elysianshadows.com/phpBB3/viewtopic.php?f=6&t=6933#p80119For me personally, I would move to Allegro 5. However, I am not a team member and I do not understand the source code all that much to begin with anyway. But from all the things I understand, Allegro 5 is the way to go. The other option would be to go the same direction of the AGS Scripting language and just have the team just create their own multimedia layer for AGS.