Make the method `backend_ops` of `DrawingArea` public[Feature Request]

### What is the feature ?
 the method `backent_ops` of `DrawingArea`

### (Optional) Why this feature is useful and how people would use the feature ?
Some crates just expose `DrawingArea` and hide it's field `backend`, so there is nothing I can do to draw a `rect` by using the pixcel coordinations.

### (Optional) Additional Information
*More details are appreciated:)*
