Update to new github.com/go-gl/mathgl package API.

gofmt -w -r 'mathgl.Vec2d -> mgl64.Vec2' ./
gofmt -w -r 'mathgl.Vec3d -> mgl64.Vec3' ./
gofmt -w -r 'mathgl.Rotate2Dd -> mgl64.Rotate2D' ./
gofmt -w -r 'mathgl.Perspectived -> mgl64.Perspective' ./
goimports -w ./