rhi: vulkan: Fix glitches on resize on X11

...by notifying QVulkanInstance about the present. With the xcb platform
this then gets turned into updating the sync request counter.

Change-Id: Iecfb6d10ead3befcb24c19433d4712ed73a84bb4
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

src/gui/rhi/qrhivulkan.cpp

@@ -1661,6 +1661,10 @@ QRhi::FrameOpResult QRhiVulkan::endFrame(QRhiSwapChain *swapChain, QRhi::EndFram
             }
         }
 
+        // Do platform-specific WM notification. F.ex. essential on X11 in
+        // order to prevent glitches on resizing the window.
+        inst->presentQueued(swapChainD->window);
+
         // mark the current swapchain buffer as unused from our side
         frame.imageAcquired = false;
         // and move on to the next buffer