bitdepth: dynamic resolution isn't supported

Hello,

Trying to change the bitdepth of a variable resolution clip is throwing this error while the format is constant:

vapoursynth.Error: bitdepth: only constant pixel formats are supported.

Looks like the is_constant_format function is looking for both variable res and variable format at the same time. Is it possible to support variable res in a future version for bitdepth and maybe other functions if it doesn't affect nor need resolution at all to work?

Repro script:

import vapoursynth as vs

clip = vs.core.std.BlankClip(None, 1920, 1080, vs.YUV420P16, varsize=True)
clip = vs.core.fmtc.bitdepth(clip, bits=10)