Add threadlock, fix event dropping on windows

Original issue: https://github.com/0xe2-0x9a-0x9b/Go-SDL/issues/25
Rewrite the import paths
Merge https://github.com/paperwing/Go-SDL
rm mixer, ttf, gfx
banish pkg-config
rm image loading
put threadlock in sdl.go
bring sdl out of subdirectory