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.

include/GLFW/glfw3.h

@@ -881,6 +881,7 @@ extern "C" {
 
 #define GLFW_NATIVE_CONTEXT_API     0x00036001
 #define GLFW_EGL_CONTEXT_API        0x00036002
+#define GLFW_OSMESA_CONTEXT_API     0x00036003
 
 /*! @defgroup shapes Standard cursor shapes
  *  @brief Standard system cursor shapes.