composite_area already honours lock transparency, but paint_gradient and
bucket_fill(_src) always passed false, and paint.bucket / paint.gradient
dropped the flag target_surface returns. Thread it through, as Edit > Fill
and the brushes already do.
Co-authored-by: prekabreki <prekabreki@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
