Blender Git Loki

VSE: Fix meta strip boundary can not be changed

In e1f3996d740c, logic for changing metastrip start and end frame based
on contained strips was removed. This was done intentionally and
incorrect functionality wasn't noticed as drawing code reflected
seemingly correct state.

Original code was mostly correct, because meta strip doesn't store its
internal start and end points. This code was restored with minor
modifications so function `SEQ_time_update_sequence()` is fully self
contained as it is used not only by transform operator.

In addition, drawing glitches that happen when meta content is outside
of meta boundaries were fixed. These glitches were not caused by
e1f3996d740c.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D11215