Blender Git Loki

Cleanup: Remove seq->tmp_flag DNA member

Commit f0d20198b290 used this field to flag rendered strips which were
queried by `SEQ_query_rendered_strips()`. Then operators iterate all
strips and checks state of `seq->tmp_flag`.

Use collection returned by `SEQ_query_rendered_strips` directly.
There should be no functional changes.

This commit adds functions `all_strips_from_context` and
`selected_strips_from_context` that provide collection of strips based
on context. Most operators can use this collection directly.
There is already `seq->tmp` DNA field, but is should not be used unless
absolutely necessary. Better option is to use human readable flag.
Best is to not use DNA fields for temporary storage in runtime.