R57.A4 fix memory leak

Fixes a memory leak (thanks @Nuevo009), and also allows vsp_text frame property to be str or bytes.

res = ...
res.akarin.Tmpl("vsp_text", "text shown in vsp status area: matrix = {{x._Matrix}}").set_output()

Requires latest https://github.com/AkarinVS/vapoursynth-plugin/releases/tag/v0.96.