Linux: use WebKit's shared-memory transport in hypervisor guests

Web views in VirtualBox (and other virtual GPUs) fail on WebKitGTK >= 2.44
because the guest GL stack cannot back the hardware DMABUF renderer. Setting
WEBKIT_DMABUF_RENDERER_FORCE_SHM=1 fixes it and keeps accelerated compositing,
confirmed in VirtualBox on WebKitGTK 2.54.

Detect a hypervisor guest from the cpuinfo hypervisor flag before GTK init and
set the variable non-replacing, so a user-exported value still wins. Bare metal
is unchanged. The skill references are updated to match.

src/libvgcode/src/Layers.cpp

@@ -75,7 +75,7 @@ std::vector<float> Layers::get_zs() const
 
 size_t Layers::get_layer_id_at(float z) const
 {
-    auto iter = std::upper_bound(m_items.begin(), m_items.end(), z, [](float z, const Item& item) { return item.z > z; });
+    auto iter = std::upper_bound(m_items.begin(), m_items.end(), z, [](float z, const Item& item) { return item.z < z; });
     return std::distance(m_items.begin(), iter);
 }