Support computing area for compound regions

The following example currently raises a `NotImplementedError`:
```python3
from regions import CirclePixelRegion, PixCoord

circle_1 = CirclePixelRegion(
center=PixCoord(0, 0),
radius=10
)

circle_2 = CirclePixelRegion(
center=PixCoord(10, 0),
radius=10
)

compound = circle_1 & circle_2
print(compound.area)
```
It would be great to have this supported!

Start with the CirclePixelRegion, PixCoord, and compound.area example in the issue, then trace how the compound region is created and where its area currently raises NotImplementedError. Done means the shown intersection computes an area instead of raising that exception, with coverage for the compound-region case.

Written by the indexing model from the issue text.