Add Vertical and Horizontal perspective sliders to Compose

The keystone existed in framing but nothing in develop could reach it:
framing is presented by its own Compose panel rather than generated, so
new framing parameters get no control until the panel names them.

Compose now has Vertical and Horizontal sliders under Straighten,
mirrored from the session like the angle, recorded as parameter steps
("Vertical Perspective" in the history), cleared by the Compose reset
and by opening the next photograph. Releasing either slider refits the
crop the way releasing the straighten slider does: a keystone alone
needs no crop, but it moves the empty corners of a straightened frame,
so the crop that avoided them before may not after, or may have room
to grow back.

ui/dr-ui/src/develop/masks.rs

@@ -225,7 +225,8 @@ impl DevelopSession {
     ///
     /// **Known gap.** A quarter turn or a flip permutes the axes, and a clip
     /// rectangle cannot express that — the straightening angle is handled
-    /// alongside this, but a quarter-turned frame shows the overlay unturned.
+    /// alongside this, but a quarter-turned frame shows the overlay unturned,
+    /// and a keystoned one (FR-DEV-20) shows it unwarped.
     /// Fixing it properly means running the overlay through the same shader
     /// prologue the image goes through, which is the right answer and a larger
     /// one than this.