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

Fix T80282: Playback start delayed with AV sync

If current frame is set to before start frame, with AV sync there is
delay when playback is started.

Tag scene with ID_RECALC_AUDIO_SEEK, so audio playback starts from
correct position.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8809

Fix T80424: Image not scaled when rendering

This is was caused by incorrectly set `preview_render_size` in VSE
rendering context. Value was set to `SEQ_PROXY_RENDER_SIZE_FULL`, but
it should be `SEQ_PROXY_RENDER_SIZE_SCENE` as scene render size is
being used.

Alternatively we can check for `context->for_render` in
`input_preprocess()`, but I think fix above is more correct.

Reviewed By: sergey, brecht

Maniphest Tasks: T80424

Differential Revision: https://developer.blender.org/D8838

VSE: make transform effect multithreaded

Multithreaded implementation can provide significant performance gain
on CPU's that can handle more parallel tasks.

On my 8 core system I got about 3.5x faster processing speed.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8585

Fix T79594: Define rounding for armature edit props

Define `RNA_def_property_ui_range` for head and tail transform properties.
Step of 10 should match original behavior when tweaking value.

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D8513

Merge branch 'blender-v2.90-release'