Add default placeholder texture to SMP interpolator

[synergiance]

People may not realize there's an ability to make a placeholder texture if there's none set by default.

This can be solved by providing a texture we put into the interpolator by default.

I've considered simply ignoring this issue and adding it to documentation, but people don't usually read documentation.

[synergiance]

Since there is now a default placeholder texture in the repo, I will close this issue.