Fix: VSE colormix blend factor not working

Blend factor was used to adjust alpha of background image, which is not
correct. This was done in fdee84fd567a where another change was, that
background alpha is copied into result, which is correct.

Apply blend factor to foreground image alpha channel.

Cleanup: Loops in VSE effect processing

Some effect functions looped over alternating lines, previously with
different factors. Since only one factor is used, code can be
simplified by looping all lines in one for loop.

There should be no functional changes.

Cleanup: Factor in VSE effect processing

2 factor variables were passed to effects, but they were hard coded to
have same value.

Remove duplicate variable from arguments, rename single argument to
`fac`. Inverted factor variables were renamed to `mfac`. Any other
factor related variables are prefixed with `temp_`.

There should be no functional changes.

Fix T94280: Crash when splitting meta strip

This happens because in `SEQ_time_update_sequence` function
`SEQ_get_meta_by_seqbase` returns uninitialized value. This isn't nice,
but it shouldn't happen in first place. Problem is, that
`SEQ_edit_strip_split` does move strips into detached `ListBase`, so
other functions can't see them anymore. Detached `ListBase` is used
solely to preserve relationships during duplication.

Move strips to original `ListBase` immediately after duplication and
return `NULL` if `SEQ_get_meta_by_seqbase` can't find meta strip.

Splitting itself can still rely on fact, that number of original and
duplicated strips is same and they are placed next to each other in
exactly same order at the end of original `ListBase`.

Fix T94254: Crash using view_all operator in VSE

Caused by `NULL` dereference in `SEQ_meta_stack_active_get()`.

Check if `Editing` is `NULL` before accessing meta stack.

Fix: Meta strip not created with alpha over blend mode

Meta add function wasn't updated. All strips now use alpha over
blending, so set it in `SEQ_sequence_alloc()`.

VSE: Clamp sound strip when adding movie strip

When adding multiple movie strips and sound stream is longer than video,
this results in gaps between video strips, which is undesirable.

Fix T93844: High memory usage during VSE preview

Since 88c02bf826df FFmpeg handles are freed if image is not displayed.
This change did not work correctly if strips are inside meta strip,
because overlap did not consider meta strip boundary, only strips inside
of meta strip.

Pass frame range to `sequencer_all_free_anim_ibufs`, if strip is inside
of meta strip, frame range is reduced to fit meta strip boundary, but if
meta strip is being edited, range must be set to +/-`MAXFRAME`,
otherwise playback performance would be too bad.

VSE: Fix strip with mask modifier not blending

Set `ibuf->planes` to `R_IMF_PLANES_RGBA` because mask modifier adds
transparent areas to image.

Fix T93314: Thumbnails not drawn with default scale

Decrease threshold for drawing thumbnails.

This was unintended change in daaa43232d34 that was overlooked.