"Rotated bounds" or "corners" attribute

Right now, `GeoTIFF` and `Overview` expose a `.bounds` attribute which gives the axis aligned bounds of the raster. But in the case of a rotated transform, this is not the same as the four corners. It would probably be nice to expose a method that returns the four corners of the image.

Here's a suggestion from claude:

```py
@property
def corners(self: HasTransform) -> tuple[tuple[float, float], tuple[float, float], tuple[float, float], tuple[float, float]]:
"""Return the four corners of the image in the units of its CRS.

Unlike `bounds`, which returns the axis-aligned bounding box, this returns
the actual image corners in order: upper-left, upper-right, lower-right, lower-left.
For non-rotated images these are equivalent; for rotated images the corners
describe the true footprint.

Returns:
(ul, ur, lr, ll) as (x, y) tuples.
"""
t = self.transform
ul = t * (0, 0)
ur = t * (self.width, 0)
lr = t * (self.width, self.height)
ll = t * (0, self.height)
return (ul, ur, lr, ll)

```

Note that the ordering of the output depends on whether the raster's origin is top left or bottom left.