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>

qml/CupWindow.qml

@@ -22,6 +22,14 @@ Window {
            | (Platform.bypassWindowManager ? Qt.X11BypassWindowManagerHint : 0)
     visible: cat.cupVisible
 
+    Timer {
+        interval: 66
+        repeat: true
+        running: Platform.shapeWindows && win.visible
+        triggeredOnStart: true
+        onTriggered: Platform.updateWindowShape(win)
+    }
+
     Cup {
         id: mug
         anchors.fill: parent