tests: start writing a test-case for exif handling
Also import parts of the exif tool, to handle saving exif into jpegs.
fix uninitialized read on error path
document an other unimplemented method
in fact GdipGetAllPropertyItems was implemented, but simply not declared in public header
tests: exercise GdipGetAllPropertyItems too
seems like saving exif into jpeg works

todo: don't leak memory, check corner cases