Use a native QVulkanWindow child for Linux DMABUF presentation instead
of making WebContentView itself a QRhiWidget. This keeps BrowserWindow
on Qt's normal QWidget backing store path while sampling compositor
DMABUF backing stores directly in Vulkan.
Keep WebContentView's paintEvent as fallback for builds without Vulkan
DMABUF support and for cases where the Vulkan instance or window cannot
be created. Set an explicit Vulkan API version before creating the
shared QVulkanInstance so validation layers do not reject Qt defaults.
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