Running with QT_VK_FORCE_STAGE_TEX does not work at all with recent
NVIDIA drivers due to QVulkanWindow's and the example's naive way of
picking the memory index. Enhance this and add a warning note to the
QVulkanWindow docs as well.
Change-Id: I7f200e11d982b56e3da3b71ee3915bd7bfca5cc1
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
1c6bd414dd