Split BKE_sequencer.h intern/extern definitions

Intern definitions are moved to sequencer/intern/sequencer.h

BKE_sequencer.h was also cleaned up a bit to make sure that functions
and structs are in correct category.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9259

Merge branch 'blender-v2.91-release'

Fix Recursion when rendering scene strip

Recursion happens in case when scene strip point to it's own scene
indirectly by using SEQ_SCENE_STRIPS option.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9264

Fix T81426: Infinite loop building VSE relations

It is possible to create scene strips pointing to each other. This is
sanitized when rendering, but in dependency graph such setup will cause
infinite loop.

This patch fixes loop in dependency graph, but same problem exists in
audaspace

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9262

Fix T81250: Crash after undoing with prefetching

Main DB and it's structs can point to different address after undoing.

In this case problem was that bmain was not updated. Same fix was
done for scene as well.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9240

Fix T68343: Rendered video plays at 600fps

Field time_base of video stream must be set for some containers,
otherwise avformat_write_header() will set it to default values.
Rendered file in such case won't be played at desired frame rate.

See init_muxer() in mux.c in ffpmeg sources.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9213

Fix T81594: Unable to reassign effect inputs

This was caused by canceling operator if strip has more than 0 inputs.
Logic should be reversed - cancel only if strip has 0 inputs.

BKE_sequencer_render_loop_check() arguments had to be sanitized because
seq_effect_find_selected() can set seq1,2,3 to NULL

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9197

Move sequencer sources from blenkernel

This is first step of refactoring task T77580.
Next step will be breaking up files into smaller ones.

Reviewed By: sergey, brecht

Differential Revision: https://developer.blender.org/D8492

Fix T80397: Select Side not working correctly

Test for side on which strip is was incorrect.

Rename eSpaceSeq_Proxy_RenderSize members

Remove word proxy from eSpaceSeq_Proxy_RenderSize members if proxy is
not used with that item. Otherwise this can be a bit confusing.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D8861