Fix address sanitizer warnings with stencil table casting

[brechtvl]

For compatibility, StencilTableReal<float> is cast to StencilTable, which has the same data layout but it's not actually an instance of that class. And so using static_cast is not strictly correct. Use reinterpret_cast instead.

[davidgyu]

Hi Brecht! Would you mind submitting an OpenSubdiv CLA which is distinct from the OpenUSD CLA we already have on file. Thanks!

[brechtvl]

Ah, I didn't realize. Sent now.

[davidgyu]

Filed as internal issue #OSD-451