Null: Add limited EGL context creation on Mesa

This provides very limited support for context creation via EGL on the
Null platform.  It supports Unix-like systems with a version of Mesa
that provides EGL_MESA_platform_surfaceless.

Even then, the actual framebuffer provided is not resized along with the
'window'.  That will hopefully change once context and framebuffer
creation are separated, but this commit should at least allow more
applications than before to run on the Null platform.

src/null_window.c

@@ -553,12 +553,15 @@ const char* _glfwGetClipboardStringNull(void)
 
 EGLenum _glfwGetEGLPlatformNull(EGLint** attribs)
 {
-    return 0;
+    if (_glfw.egl.EXT_platform_base && _glfw.egl.MESA_platform_surfaceless)
+        return EGL_PLATFORM_SURFACELESS_MESA;
+    else
+        return 0;
 }
 
 EGLNativeDisplayType _glfwGetEGLNativeDisplayNull(void)
 {
-    return 0;
+    return EGL_DEFAULT_DISPLAY;
 }
 
 EGLNativeWindowType _glfwGetEGLNativeWindowNull(_GLFWwindow* window)