Picture film inside a mask layer

The manual described film in a mask layer (6b99f67) with no picture of
it, and the paragraph named the control `Print exposure` where the
panel says `Print Exposure`.

A new scene, film_local, sets Kodak Ektar 100 on an upright alpine
frame — a negative, so it is printed and has a print exposure — then
adds a linear gradient, turns it by its rotate handle to fall from the
top, moves it onto the sky, shows the mask as its edge so the print
stays visible, and raises the layer's Print Exposure to burn the sky
in. Done Masking, then Before held. It puts the masks back to Tint
before undoing, as the other scenes expect.

The picture follows the film list's, under the paragraph it
illustrates. The bundled manual is regenerated to match.

docs/manual/README.md

@@ -273,13 +273,16 @@ sensor does not. The list opens over the column and scrolls on its own — by
 wheel, drag or flick, or with `Up`, `Down` and `Enter` — down to the
 black-and-white stocks at its end.
 
+![Opening the film list, scrolling it, choosing Velvia, then holding Before](media/film.gif)
+
 The film's sliders work on a mask as they do on the whole photograph, the way
-a printer dodges and burns: in a layer, `Print exposure` darkens or lightens
+a printer dodges and burns: in a layer, `Print Exposure` darkens or lightens
 that region of the print, `Push` develops it further, and the stock stays the
 one the photograph was made on. Where layers overlap, the photograph takes the
-average of what they ask for.
+average of what they ask for. Below, a negative stock on an alpine frame, then
+a gradient over the sky whose `Print Exposure` burns it in.
 
-![Opening the film list, scrolling it, choosing Velvia, then holding Before](media/film.gif)
+![Kodak Ektar 100 on the whole frame, a gradient turned to cover the sky, its Print Exposure raised to burn the sky in, then holding Before](media/film-local.gif)
 
 ### History, snapshots, presets