Use dep to vendor things

vendor/github.com/go-gl/mathgl/mgl32/doc.go

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+// Copyright 2014 The go-gl Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package mgl[32|64] (an abbreviation of mathgl since the packages were split between 32 and 64-bit versions)
+is a pure Go math package specialized for 3D math, with inspiration from GLM. It provides statically-sized vectors and matrices with
+compile-time generated calculations for most basic math operations. It also provides several basic graphics utilities such as bezier curves and surfaces,
+generation of basic primitives like circles, easy creation of common matrices such as perspective or rotation, and common operations like converting
+to/from screen/OpenGL coordinates or Projecting/Unprojecting from an MVP matrix. Quaternions are also supported.
+
+The basic vectors and matrices are written with code generation, so looking directly at the source will probably be a bit confusing. I recommend looking at the Godoc
+instead, as all basic functions are documented.
+
+This package is written in Column Major Order to make it easier with OpenGL. This means for uniform blocks you can use the default ordering, and when you call
+pass-in functions you can leave the "transpose" argument as false.
+
+The package now contains variable sized vectors and matrices. Using these is discouraged. They exist for corner cases where you need "small" matrices that are still
+bigger than 4x4. An example may be a Jacobean used for inverse kinematics. Things like computer vision or general linear algebra are best left to packages
+more directly suited for that task -- OpenCV, BLAS, LAPACK, numpy, gonum (if you want to stay in Go), and so on.
+*/
+package mgl32