Shape the window when there is no compositor

On Xfce with compositing off, X11 has no alpha to blend against, so the
transparent background came out as a black box around the cat.

When a window comes up without an alpha channel, TNCat now cuts the window down
to the pixels the drawing covers, using the X11 shape extension, and refreshes
that 15 times a second so it follows the animation. A sitting cat in a 120 x 106
window comes out as 96 rectangles over 29 percent of the window, so most of the
box is gone. Edges are harder than with a compositor, one bit per pixel, and the
log says so and names the Xfce setting that gives smooth ones instead.

The image-to-rectangles step is platform independent and folds every device row
into its window row, which both halves the rectangle count on a scaled display
and keeps a pixel that is opaque in only one of them.

xcb and xcb-shape are picked up through pkg-config on Unix and the build still
works without them. Opaque mode now paints a neutral card rather than the coat
colour.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -102,9 +102,21 @@ This is the main target. Two things to know.
 **Transparency needs a compositor.** Without one, X11 has no alpha channel to
 composite against and the transparent window background renders as a black box
 around the cat. In Xfce it is one checkbox: Settings, Window Manager Tweaks,
-Compositor, Enable display compositing. TNCat prints a warning naming that
-setting when its window comes up without an alpha channel. `TNCAT_OPAQUE=1`
-paints a solid background instead, which is usable but not pretty.
+Compositor, Enable display compositing. That is the best answer, because alpha
+gives smooth edges.
+
+When TNCat sees a window come up without an alpha channel it says so and falls
+back to shaping the window with the X11 shape extension: the window is cut down
+to the pixels the cat covers, so there is no box left to be black. The shape is
+one bit per pixel, so edges are harder than with a compositor, and it is
+refreshed 15 times a second to follow the animation. Measured on a sitting cat
+in a 120 x 106 window, the shape is 96 rectangles covering 29 percent of the
+window.
+
+Shaping needs `xcb` and `xcb-shape` at build time (`libxcb-shape0-dev` on
+Debian and Ubuntu, `xcb-util-devel` on SUSE). CMake says so if they are
+missing, and the build still works without them. `TNCAT_OPAQUE=1` skips the
+whole question and paints a solid background.
 
 **Keeping the cat on top.** The default is a frameless `Qt::Tool` window that
 stays on top. If the window manager puts it behind other windows or shows it in