backends/{x11,virtual}: Allocate render data for all available outputs

This makes the behavior of virtual and x11 backend consistent
with the drm and the wayland backend and reduces the number of users of
the Platform::outputEnabled signal.

src/backends/virtual/virtual_egl_backend.cpp

@@ -122,13 +122,11 @@ void VirtualEglBackend::init()
 
     const auto outputs = m_backend->outputs();
     for (Output *output : outputs) {
-        if (output->isEnabled()) {
-            addOutput(output);
-        }
+        addOutput(output);
     }
 
-    connect(m_backend, &VirtualBackend::outputEnabled, this, &VirtualEglBackend::addOutput);
-    connect(m_backend, &VirtualBackend::outputDisabled, this, &VirtualEglBackend::removeOutput);
+    connect(m_backend, &VirtualBackend::outputAdded, this, &VirtualEglBackend::addOutput);
+    connect(m_backend, &VirtualBackend::outputRemoved, this, &VirtualEglBackend::removeOutput);
 }
 
 bool VirtualEglBackend::initRenderingContext()

src/backends/virtual/virtual_qpainter_backend.cpp

@@ -47,14 +47,12 @@ VirtualQPainterBackend::VirtualQPainterBackend(VirtualBackend *backend)
     : QPainterBackend()
     , m_backend(backend)
 {
-    connect(backend, &VirtualBackend::outputEnabled, this, &VirtualQPainterBackend::addOutput);
-    connect(backend, &VirtualBackend::outputDisabled, this, &VirtualQPainterBackend::removeOutput);
+    connect(backend, &VirtualBackend::outputAdded, this, &VirtualQPainterBackend::addOutput);
+    connect(backend, &VirtualBackend::outputRemoved, this, &VirtualQPainterBackend::removeOutput);
 
     const auto outputs = backend->outputs();
     for (Output *output : outputs) {
-        if (output->isEnabled()) {
-            addOutput(output);
-        }
+        addOutput(output);
     }
 }
 

src/backends/x11/windowed/x11_windowed_qpainter_backend.cpp

@@ -52,13 +52,11 @@ X11WindowedQPainterBackend::X11WindowedQPainterBackend(X11WindowedBackend *backe
 {
     const auto outputs = m_backend->outputs();
     for (Output *output : outputs) {
-        if (output->isEnabled()) {
-            addOutput(output);
-        }
+        addOutput(output);
     }
 
-    connect(backend, &X11WindowedBackend::outputEnabled, this, &X11WindowedQPainterBackend::addOutput);
-    connect(backend, &X11WindowedBackend::outputDisabled, this, &X11WindowedQPainterBackend::removeOutput);
+    connect(backend, &X11WindowedBackend::outputAdded, this, &X11WindowedQPainterBackend::addOutput);
+    connect(backend, &X11WindowedBackend::outputRemoved, this, &X11WindowedQPainterBackend::removeOutput);
 }
 
 X11WindowedQPainterBackend::~X11WindowedQPainterBackend()