Fix IsWindowFocused() on web.

[marionauta]

Currently the web platform uses glfwSetWindowFocusCallback which seems to do nothing regarding the game canvas' focus. This PR adds emscripten focus/blur callbacks targeting the canvas.

Sample project focus.zip

#include <raylib.h>
#include <emscripten/emscripten.h>

void gameupdate(void) {
    BeginDrawing();
    ClearBackground(WHITE);
    if (IsWindowFocused()) {
        DrawText("Focused", 10, 10, 50, BLACK);
    } else {
        DrawText("Unfocused", 10, 10, 50, BLACK);
    }
    EndDrawing();
}

int main(void) {
  InitWindow(400, 400, "focus events");
  emscripten_set_main_loop(gameupdate, 0, 1);
  CloseWindow();
  return 0;
}

Screen.Recording.2024-12-26.at.19.08.47.mov

[raysan5]

@marionauta Thanks for the improvement! As per the sources, it seems the function is set but I can't see how it is used...