fix(examples): keep the glfw demo's region off the pan path, and print the multiplot's region geometry

Adding a grabbable region across the middle of the glfw demo turned the pan
gesture PLOT_DEMO_FOLLOW makes -- and a user's own centre drag -- into a region
drag, which that mode's self-check reported as 'distance behind only by the pan:
NO' and exit 1. The region now rides the left sixth of the live window, so a
press in the middle pans exactly as it always did, and the reason is written
where the bounds are placed.

The multiplot demo also prints its region's last-frame fill rect and bound
columns, so the exit frame's region pixels can be read against the geometry
Plot::layout() measured.

Report: .superpowers/sdd/p2-task-7-report.md

examples/multiplot_demo.cpp

@@ -655,6 +655,17 @@ int main() {
               " %.4f, max %.4f, integral %.4f\n",
               region, rs.x0, rs.x1, static_cast<unsigned long long>(rs.count), rs.mean, rs.rms,
               rs.min, rs.max, rs.integral);
+  // The region's last-frame geometry on row 0, as the layout measured it: the rect the
+  // fill and the bound strokes are drawn in, which is also what the screenshot's region
+  // pixels are read against.
+  for (const plot::RegionLayout& rl : std::as_const(figure.plot(0)).layout().regions) {
+    if (rl.region != region) continue;
+    std::printf("[multiplot] region       %u: fill x %.1f y %.1f w %.1f h %.1f, bounds at %.1f and"
+                " %.1f\n",
+                region, rl.fill.x, rl.fill.y, rl.fill.w, rl.fill.h,
+                static_cast<plot::f64>(plot::handle_column(rl.left_handle)),
+                static_cast<plot::f64>(plot::handle_column(rl.right_handle)));
+  }
   std::printf("[multiplot] cursors      a %.6f s, b %.6f s, dx %g s dy %g\n",
               figure.plot(0).cursor(cursor_a).x, figure.plot(0).cursor(cursor_b).x,
               figure.plot(0).measurement(measure).dx, figure.plot(0).measurement(measure).dy);