VSE doesn support pure emissive colors when rendering, but weren't
showed inside the preview. This was a limitation when the background
checkerbord is draw. It renders the alpha of the overlay to 1.0 making
the background be drawn on top of the image, hiding the pure emissive
colors.
This PR fixes this by clearing the alpha. This will render the
checkerboard below the image.
Another solution would be to set the alpha to zero in the theme, but that
adds confusion as theme colors will still be able to alter the support of
pure emissive colors.
Pull Request: https://projects.blender.org/blender/blender/pulls/146562
c279d894db