Add OS X support to helper package.

Simply reuse the linux version for both linux and darwin via build constraints.
Move `func TextureFromDDS(fname string) (gl.Texture, error)` into dds.go, making it available on Windows, Linux, OS X. I've tested it on OS X and it seemed to work.

Fixes the following issue on OS X:

```
$ go get -d -u github.com/go-gl/mathgl/...
package github.com/go-gl/mathgl/examples/opengl-tutorial/indexer
	imports github.com/go-gl/mathgl/mgl32
	imports github.com/go-gl/mathgl/examples/opengl-tutorial/input
	imports github.com/go-gl/glfw3
	imports github.com/go-gl/mathgl/mgl64
	imports github.com/go-gl/mathgl/examples/opengl-tutorial/objloader
	imports github.com/go-gl/mathgl/examples/opengl-tutorial/tutorial01
	imports github.com/go-gl/gl
	imports github.com/go-gl/mathgl/examples/opengl-tutorial/tutorial02
	imports github.com/go-gl/mathgl/examples/opengl-tutorial/helper
	imports github.com/go-gl/mathgl/examples/opengl-tutorial/helper
	imports github.com/go-gl/mathgl/examples/opengl-tutorial/helper: no buildable Go source files in /Users/Dmitri/Dropbox/Work/2013/GoLand/src/github.com/go-gl/mathgl/examples/opengl-tutorial/helper
```