create a field_offset macro that takes a structure type and a field and
returns the offset of the field within the structure
everything else:
use field_offset to calculate the size of variable sized structs
r_local.h, r_shared.h, client.h and render.h at the same time (couldn't get
away from that:/) verly likely to be lots of breakage (eg, player and eye
models are NOT checksummed atm), but everything builds
