Support finding SDL2 and minimal build fixes.
Start working on adapting graphic::Manager. [noci]
Compiles succesfully with SDL2! [ci skip]
Include SDl2 source in the externals dir.
Support for building SDL2 image/mixer.

Compilation flags are inexistent, no images/music can be loaded. (except, possibly, for WAVE music)
Build and warning fixes.
Build SDL2-image with support for some file formats.

Also, some refactoring in include_dirs and don't request DirectX for SDL.
Better error message for texture loading failure.
Better error messages for graphic initialization failures.
Add libpng and zlib to the repository.
Fixing embeded libpng.
Fix input.
Travis update
Merge branch 'sdl2' into dev