Merge pull request #15 from PureSwift/feature/sdl1

Add SDL 1.2 wrappers

Merge pull request #12 from PureSwift/feature/image-mixer

Add SDL_image/SDL_mixer support

Merge pull request #10 from PureSwift/feature/sdl3

Add SDL3 support

Update example app

Ignore SwiftPM files

Fixed for Swift 3.0.2 compatibility

minor optimization