Make OSMesa backend an almost proper null backend

It still depends on POSIX time and TLS.

src/null_monitor.c

@@ -0,0 +1,55 @@
+//========================================================================
+// GLFW 3.3 - www.glfw.org
+//------------------------------------------------------------------------
+// Copyright (c) 2016 Google Inc.
+// Copyright (c) 2006-2016 Camilla Löwy <elmindreda@glfw.org>
+//
+// This software is provided 'as-is', without any express or implied
+// warranty. In no event will the authors be held liable for any damages
+// arising from the use of this software.
+//
+// Permission is granted to anyone to use this software for any purpose,
+// including commercial applications, and to alter it and redistribute it
+// freely, subject to the following restrictions:
+//
+// 1. The origin of this software must not be misrepresented; you must not
+//    claim that you wrote the original software. If you use this software
+//    in a product, an acknowledgment in the product documentation would
+//    be appreciated but is not required.
+//
+// 2. Altered source versions must be plainly marked as such, and must not
+//    be misrepresented as being the original software.
+//
+// 3. This notice may not be removed or altered from any source
+//    distribution.
+//
+//========================================================================
+
+#include "internal.h"
+
+
+//////////////////////////////////////////////////////////////////////////
+//////                       GLFW platform API                      //////
+//////////////////////////////////////////////////////////////////////////
+
+void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)
+{
+}
+
+GLFWvidmode* _glfwPlatformGetVideoModes(_GLFWmonitor* monitor, int* found)
+{
+    return NULL;
+}
+
+void _glfwPlatformGetVideoMode(_GLFWmonitor* monitor, GLFWvidmode* mode)
+{
+}
+
+void _glfwPlatformGetGammaRamp(_GLFWmonitor* monitor, GLFWgammaramp* ramp)
+{
+}
+
+void _glfwPlatformSetGammaRamp(_GLFWmonitor* monitor, const GLFWgammaramp* ramp)
+{
+}
+