MoltenVK on Qt 6

- Install MoltenVK

   1) brew install molten-vk

   or	

   2) Build from source: https://github.com/KhronosGroup/MoltenVK
	
- Set VULKAN_SDK

cmake looks for $VULKAN_SDK/include/vulkan/vulkan.h, so you should set
VULKAN_SDK accordingly. For example, when installing MoltenVK from brew
the paths are:

    export VULKAN_SDK=/usr/local/Cellar/molten-vk/1.1.5/libexec/    (intel)
    export VULKAN_SDK=/opt/homebrew/Cellar/molten-vk/1.1.5/libexec/ (ARM)

- Apply build system patch, or change MoltenVK includes

https://codereview.qt-project.org/c/qt/qtbase/+/376235/

Alternatively, three files in qtbase/src/plugins/platforms/cocoa needs to be changed:

   -#include <MoltenVK/mvk_vulkan.h>
   +#include <../../include/MoltenVK/mvk_vulkan.h>

- Configure and build Qt as usual.

Check that it says “Vulkan Yes” somewhere in the configure output.

- Set QT_VULKAN_LIB to point to libMoltenVK.dylib

    (homebrew paths again)

    export QT_VULKAN_LIB=/usr/local/Cellar/molten-vk/1.1.5/lib/libMoltenVK.dylib     (intel)
    export QT_VULKAN_LIB=/opt/homebrew/Cellar/molten-vk/1.1.5/lib/libMoltenVK.dylib  (ARM)
 
- Set QSG_RHI_BACKEND=vulkan

    QSG_RHI_BACKEND=vulkan qtquick3d/examples/quick3d/helloqtquick3d/helloqtquick3d.app/Contents/MacOS/helloqtquick3d