Add GLFW_OSMESA_CONTEXT_API

This allows the creation of OpenGL contexts via OSMesa on existing
platforms.  It does not add a compile- or link-time dependency on
OSMesa.

Fixes #281.

src/x11_platform.h

@@ -100,6 +100,7 @@ typedef VkBool32 (APIENTRY *PFN_vkGetPhysicalDeviceXcbPresentationSupportKHR)(Vk
 #include "xkb_unicode.h"
 #include "glx_context.h"
 #include "egl_context.h"
+#include "osmesa_context.h"
 #if defined(__linux__)
 #include "linux_joystick.h"
 #else