add wrap mode arg to st_warp

**Is your feature request related to a problem? Please describe.**
The ImageBufAlgo.warp function provides the power to control the wrap mode - i.e. how to generate pixel values outside of the image's data window.
However, st_warp doesn't provide this - or at least doesn't via the Python bindings.

**Describe the solution you'd like**
Add warp mode to the ImageBufAlgo.st_warp function

**Describe alternatives you've considered**
I was wondering about giving access to the Filter2D class in Python, or some new class which represents the combination of a filter and some other options like wrap mode. For any functions which use these options you just pass in an object holding all the option values.

**Additional context**
While comparing warped images made by OIIO vs. Nuke, the former has darker edges, because black outside is the default for st_warp.

Start with the ImageBufAlgo.st_warp entry point and its Python bindings, then compare how ImageBufAlgo.warp exposes wrap mode. The work is done when st_warp accepts a wrap-mode option and the resulting edge pixels no longer default to black when another mode is selected.

Written by the indexing model from the issue text.