Add function to create image from spritesheet rect

This would avoid the need to repeatedly use the idiom of:

```cpp
Image* image = create_image(graphics_ptr, SIZE);
image->set_render_target();
spritesheet->rect(Rect(IMAGE_LOCATION, SIZE), Vec(0,0), 1.0f);
```

And instead just do:
```cpp
Image* image = spritesheet->image_from_rect(Rect(IMAGE_LOCATION, SIZE), 1.0f); // 1.0f is the scale
```