Searches for a StoredProperty that is valid in the given context.
Property chunks are automatically stored by ParseIFF() when pre-declared
by PropChunk() or PropChunks(). The returned storedproperty contains
a pointer to the data in the chunk.
INPUTS
iff
a pointer to a an IFFHandle struct.
type
type code of property to search for.
id
id code of property to search for.
RESULT
sp - pointer to a storedproperty if found, NULL if none are found.