diag: WOWEE_WIDGET_DUMP=3 outlines every widget where it thinks it is

Everything says the main bar should be on screen — laid out centred at the
bottom, the right size, its art resident on the GPU, alpha 1, visible — and
it is not there. Text and filled rectangles from this same layer have been
visible all along; no texture from it ever has.

Outlining each widget tells the two remaining possibilities apart by
looking: outlines but no art means the images are the problem, and no
outlines means the layer is being covered or discarded. Either beats another
round of inference from a downscaled screenshot.

src/ui/widget_renderer.cpp

@@ -375,6 +375,15 @@ void WidgetRenderer::render(WidgetTree& tree, float screenW, float screenH) {
             continue;
         }
 
+        // Level 3 outlines every widget where it believes it is. If the
+        // outlines appear and the art does not, the images are the problem; if
+        // neither appears, the whole layer is being covered or discarded. That
+        // is two possibilities told apart by looking, rather than inferred from
+        // a screenshot.
+        if (dumpWidgets >= 3) {
+            dl->AddRect(ImVec2(x0, y0), ImVec2(x1, y1), IM_COL32(255, 0, 255, 200));
+        }
+
         if (w->kind == WidgetKind::Texture) {
             // A colour set with SetTexture(r,g,b) fills; a texture with no
             // file at all draws nothing. Treating the second as the first