more jpeg read sanity checking
foreign memory images tagges as ANY

we weren't tagging foreign memory buffers as supporting ANY demand
style, so they were falling back to SMALLTILE

this broke any operation using SEQUENTIAL_UNBUFFERED plus foreign
buffers, eg. falsecolour, since UNBUF needs very strict seq access
falsecolour failed for some image types

type prevented the auto-cast-first-band thing working
support many Radiance readers active at once

we had some globals for the Rad reader state, whcih was fine with the
old reader, but with the new sequential one you can have several active
at once