buffer: make data private

All image related data are specific to the IMAGE payload. It's better
to access them using accessors, which allows to emit warnings in case
of payload type mismatch.
camera: add chunk data API
documentation: new buffer API documentation
build: add missing arvbufferprivate.h header as noinst_HEADERS
tests: build with -Wall
viewer: remove -lm from AM_CFLAGS

It should be set by configure.