Blender Git Loki

Fix T83266: Proxy building with meta strips fails

Building proxy with meta strip selected resulted in attempt to render
meta strip itself and save result as image as is done when building
image proxies.

Remove meta strip from list of supported strips.

Multicam, Meta and scene strip are cleared from poll function in UI as well.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D10001