Why CanvasRenderingContext2d.get_image_data argument is float?

### Summary

https://rustwasm.github.io/wasm-bindgen/api/web_sys/struct.CanvasRenderingContext2d.html#method.put_image_data
```rs
pub fn get_image_data(
    &self,
    sx: f64,
    sy: f64,
    sw: f64,
    sh: f64{
) -> Result<ImageData, JsValue>
```
webidl's difinition?
I think not bad u32 or i32 data format.