SEQ_modifier_apply_stack is called from exactly one place, and it always
was throwing away the input image, and taking the newly produced image.
Which means, we can stop copying it. Just run the modifiers on the
input image.
In a test file that is HD (1080p) resolution, two EXR image sequences,
adjustment layer with Color Balance, and a text strip on top,
playback framerate (Ryzen 5950X, Win10/VS2022) goes 20.8 -> 22.1 fps
Pull Request: https://projects.blender.org/blender/blender/pulls/127346
577630d24f