UI: Fix stats window

[deltamaya]

Now the frame count in stats window will display correctly with rescale output turned on
I think this time it makes more sense?

Description

Use the encoder's video frame count instead of the obs main mix video

Motivation and Context

Fixes issue: #10744

How Has This Been Tested?

Tested with simple scene when streaming and recording

Types of changes

Bug fix

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[norihiro]

The commit description has a too-long line. It should be wrapped at 72-column.