util img: small fixes

don't claim we support subtraction of integer types
util img_fast: small optimisation if tint and many white or black pixels

multiplication can be skipped in these cases. It saves 50% time for each extreme pixel.
model: add MD_DIMS metadata

allows to specify the format of the data, especially handy for RGB image.
hdf5: handle RGB data better

If data is explicitly RGB (CYX or YXC), then save as CYX, and read it back correctly.

That allows to actually be able to see the image in hdfview, and detect when reading back that
it is a RGB data.
It doesn't work in Huygens... but almost could.

Also restrict further the grouping of data to only group over channel.
That's only case where that matters, and it avoids trying to be too clever with data given.
driver simcam: make it working

basically working, with test cases.
Fake image acquired from a Phenom overview camera (RGB)
driver tests: small clean up to test cases