testshade/testrender : have get_attribute fall back on get_userdata,

testshade/testrender : have get_attribute fall back on get_userdata,
and have get_userdata bind "s" and "t" (to u & v).

This lets us simulate and test (in testshade & testrender) retrieval of
userdata, and also illustrates more clearly how we assume that get_attribute
will allow binding to userdata, if present.