Fix overflow in `blit_bitmap`

[x3ro]

Hey folks, thanks for building this project 🦀

I ran into an index out of bounds error when playing around with the blit_bitmap example when using the SvgBackend instead of the BitmapBackend:

1. blit_bitmap receives dimensions iw, ih as parameters, but also calls get_size() to retrieve the size of the DrawingBackend
2. It then proceeds to use w, h to check whether or not the image is being drawn out of bounds (I think)
3. However, it appears that there was a mixup when implementing the image data access, where w was used instead of iw

The attached change fixes this, though I'm not entirely sure whether the fix takes everything into account 🤔