Add headless OSMesa backend

Allows creation and drawing to in-memory OpenGL contexts.

This backend does not provide input.

Related to #850.

src/osmesa_platform.h

@@ -0,0 +1,61 @@
+
+#ifndef _osmesa_platform_h_
+#define _osmesa_platform_h_
+
+#include <dlfcn.h>
+
+#define _GLFW_PLATFORM_WINDOW_STATE         _GLFWwindowOSMesa     osmesa
+#define _GLFW_PLATFORM_MONITOR_STATE        _GLFWmonitorOSMesa    osmesa
+#define _GLFW_PLATFORM_CURSOR_STATE         _GLFWcursorOSMesa     osmesa
+
+#define _GLFW_PLATFORM_LIBRARY_WINDOW_STATE _GLFWwinlibraryOSMesa osmesawin
+
+#define _GLFW_PLATFORM_LIBRARY_JOYSTICK_STATE
+#define _GLFW_EGL_CONTEXT_STATE
+#define _GLFW_EGL_LIBRARY_CONTEXT_STATE
+
+#include "osmesa_context.h"
+#include "posix_time.h"
+#include "posix_tls.h"
+
+#define _glfw_dlopen(name) dlopen(name, RTLD_LAZY | RTLD_LOCAL)
+#define _glfw_dlclose(handle) dlclose(handle)
+#define _glfw_dlsym(handle, name) dlsym(handle, name)
+
+// TODO(dalyj) "PLACEHOLDER" variables are there to silence "empty struct"
+// warnings
+
+// OSMesa-specific per-window data
+//
+typedef struct _GLFWwindowOSMesa
+{
+  int width;
+  int height;
+} _GLFWwindowOSMesa;
+
+
+// OSMesa-specific global data
+//
+typedef struct _GLFWwinlibraryOSMesa
+{
+  int PLACEHOLDER;
+} _GLFWwinlibraryOSMesa;
+
+
+// OSMesa-specific per-monitor data
+//
+typedef struct _GLFWmonitorOSMesa
+{
+  int PLACEHOLDER;
+} _GLFWmonitorOSMesa;
+
+
+// OSMesa-specific per-cursor data
+//
+typedef struct _GLFWcursorOSMesa
+{
+  int PLACEHOLDER;
+} _GLFWcursorOSMesa;
+
+
+#endif // _osmesa_platform_h_