When doing average/min/max luminance calculations for tonemapping, ignore
pixels that are outside of the strip rectangle. Due to how VSE innards work,
when a strip is positioned to not cover the whole screen, "the rest" is filled
with transparent black. For tonemapping, this was dragging average calculated
luminance way down.
Images of the issue in PR.
Pull Request: https://projects.blender.org/blender/blender/pulls/127207
78972f8559