Renamed `iam.c' to  `db.c'

Since it will contain a bunch of `asset_Database()' related code,
not just the IAM loader.
Fixed two little issues with the build.

Missing `#include <setjmp.h>' from `grafix/methods.h', and forgot
to rename `iam.c' to `db.c' in `asset/Makefile'.
Implemented `asset_GetTexture()'

Also added a basic test for it to `main()'.