Add a note when working directory is clean.
Update to new github.com/go-gl/mathgl/... package API.

Breaking API changes can be a good thing. You get a better API, and
updating is as easy as:
gofmt -w -r 'mathgl.Vec2d -> mgl64.Vec2' ./
gofmt -w -r 'mathgl.Vec3d -> mgl64.Vec3' ./
gofmt -w -r 'mathgl.Mat4d -> mgl64.Mat4' ./
gofmt -w -r 'mathgl.Rotate2Dd -> mgl64.Rotate2D' ./
gofmt -w -r 'mathgl.Perspectived -> mgl64.Perspective' ./
gofmt -w -r 'mathgl.Projectd -> mgl64.Project' ./
goimports -w ./
Thanks @Jragonmiris!