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.
Clarifying my comment above: "these" refers to `emscripten_set_canvas_css_size` and `emscripten_get_canvas_css_size`, since those can be fractional. `emscripten_set_canvas_size` and `emscripten_get_canvas_size` will always be integer.