emscripten: Add a hint for specifying the canvas selector

Now that we're not going through EGL, this is easy

include/SDL_hints.h

@@ -366,6 +366,15 @@ extern "C" {
  */
 #define SDL_HINT_EMSCRIPTEN_ASYNCIFY   "SDL_EMSCRIPTEN_ASYNCIFY"
 
+/**
+ *  \brief Specify the CSS selector used for the "default" window/canvas
+ *
+ * This hint only applies to the emscripten platform
+ *
+ * The default value is "#canvas"
+ */
+#define SDL_HINT_EMSCRIPTEN_CANVAS_SELECTOR "SDL_EMSCRIPTEN_CANVAS_SELECTOR"
+
 /**
  *  \brief override the binding element for keyboard inputs for Emscripten builds
  *