to avoid undefined symbol errors in certain uses of the library

to allow access for pickling in pyapr

to avoid overflows when the input image has values near the largest
representable number. New behavior is that no offset is added when
either a) `lambda<=0` or b) the image covers the entire range of values.

required for `FetchContent_MakeAvailable` when building gtest

If not externally available, gtest is downlaoded and built using
CMake's FetchContent

Joel tmp