Add headless OSMesa backend

Allows creation and drawing to in-memory OpenGL contexts.

This backend does not provide input.

Related to #850.

src/internal.h

@@ -172,6 +172,8 @@ typedef void (APIENTRY * PFN_vkVoidFunction)(void);
  #include "wl_platform.h"
 #elif defined(_GLFW_MIR)
  #include "mir_platform.h"
+#elif defined(_GLFW_OSMESA)
+ #include "osmesa_platform.h"
 #else
  #error "No supported window creation API selected"
 #endif