backends/virtual: Provide vertically mirrored layer buffers

This makes the behavior of the virtual backend more consistent with
wayland, x11, and drm backends. However, the main advantage is that it's
more nicer to memfd fallback code path in screencasting plugin.

src/backends/virtual/virtual_egl_backend.cpp

@@ -46,6 +46,7 @@ std::optional<OutputLayerBeginFrameInfo> VirtualEglLayer::beginFrame()
     if (!m_texture || m_texture->size() != nativeSize) {
         m_fbo.reset();
         m_texture = std::make_unique<GLTexture>(GL_RGB8, nativeSize);
+        m_texture->setContentTransform(TextureTransform::MirrorY);
         m_fbo = std::make_unique<GLFramebuffer>(m_texture.get());
     }