_splat.c: add _splat.spline_value function
splat.interpol.PolyList.value: use _splat.spline_value
splat.interpol.PolyList: use _splat.Spline
test.py: add signal test cases
bm.py: add spline.value case + fix Spline pts
_splat.c: avoid unnecessary calls to find_spline_poly
bm.py: test_offset_frag: do not use Fragment.dup
_splat.c: splat_find_spline_poly: add type checks