Move getRotation and getImageArea to the base class

This way we know we can always safely call getRotation on any image.

In particular, PagedTiffHandler doesn't extend BitmapHandler, and I
want to make VipsScaler work with PagedTiffHandler, and it calls
getRotation.

Change-Id: I63f2be2a6f31398918b8562e15343f8a839d91c1
Merge "Move getRotation and getImageArea to the base class"