Fix misaligned LV2 Atoms

ubsan detects misaligned reads/writes on lv2 atom ports. This adds _alignment_padding to _LV2_Atom_Buffer. from, https://lv2plug.in/ns/ext/atom "An LV2_Atom has a 32-bit size and type, followed by a body of size bytes. Atoms MUST be 64-bit aligned."