Move image.go into separate package
Export sdl.GetError
Update references to GetError
Expose InternalSurface to allow creating surfaces from image package
Don't require SDL2_image library in sdl package
Remove redundant cgo declarations
Add showimage.go demo
Add gitignore for demos/showimage
Merge with upstream master
Merge pull request #9 from zombiezen/imagepkg

Separate image.go into an image package