Feature Request: USE_GLFW + PROXY_TO_PTHREAD + OFFSCREENCANVAS_SUPPORT

**Overview**

Current version of emscripten seems not to support `USE_GLFW + PROXY_TO_PTHREAD + OFFSCREENCANVAS_SUPPORT` linker flang combination.

**Current Status**
The application built with the flags, `USE_GLFW + PROXY_TO_PTHREAD`, will succeed, but gives following error:

Code:
```
function _glfwInit() {
// ...
window.addEventListener("gamepadconnected", GLFW.onGamepadConnected, true); // this line
}
```

Error:
```
window is not defined
```

Almost functions of GLFW can be proxied to the main thread. However, with the additional flag `OFFSCREENCANVAS_SUPPORT`, another runtime error occurs:

```
Cannot set width of placeholder canvas transferred to OffscreenCanvas.
```

I think this error means that canvas size modification should be done in the worker thread.