Blender Git Loki

VSE: Fix prefetch initialization sequence

In 8d1978a8e013 bmain entry is updated when prefetching is started,
but this must be done before seq_prefetch_update_context(). Otherwise
created cache keys will be incorrect.

This happens immediately after prefetch is started. When it's refreshed
problem goes away.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9581