add api useful for handling HiDPI

With this you can do something like:
```
double ratio = emscripten_get_device_pixel_ratio();
emscripten_set_canvas_size(w * ratio, h * ratio);
emscripten_set_canvas_css_size(w, h);
```
to get HiDPI webgl.
I see there is no error handling here, and the return types are `void`. If the `target` is not found here, this will abort the whole application. The functions in html5.h have an `EMSCRIPTEN_RESULT` result type, and will return `EMSCRIPTEN_RESULT_UNKNOWN_TARGET` if the target is not valid. I think we should do that here as well to make this robust against errors.