Blender Git Loki

Cleanup: Remove goto statements from strip rendering functions

Remove goto statement from `seq_render_image_strip()` and `seq_render_movie_strip()`.
`seq_render_image_strip_view()` and `seq_render_movie_strip_view()` is used to render individual views in both monoview and multiview branch.

I have included `seq_can_use_proxy()` for convinience

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D7870