Screenshots rendered the terminal content with transparent margins and
uncovered areas. Composite the session's real background behind the
content so the result is opaque and matches what's on screen: a
background image with its scaling mode and default-background blend, or
the solid background color when there's no image. Window transparency is
deliberately ignored since a saved screenshot has nothing behind it.

The background is laid out as though the pane were the size of the
screenshot (each multi-part piece gets its own), and is rendered one
slice at a time so memory stays bounded the way the streaming encoder
already is. Slices are produced on the main thread; compositing stays
off-main via CGContext. Applied at every output path: the standard
save/copy render, the streaming/multi-part encoder, and the live
preview.

The preview also shows a checkerboard in the area outside the image so
the boundary of what will be saved is clear when the content is smaller
than the preview.