glfw: add script to fetch upstream sources

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

update-upstream.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -ex
+
+rm -rf upstream/
+mkdir upstream/
+cd upstream/
+git clone https://github.com/glfw/glfw
+cd glfw/
+git checkout 3.3.4
+
+# Remove non-C files
+rm -rf .appveyor.yml .git .gitattributes .gitignore .mailmap .travis.yml
+rm cmake_uninstall.cmake.in README.md
+rm -r CMake* deps/ examples/ tests/ docs/
+rm src/CMakeLists.txt src/*.in