Limit the device access memory to 4 GB

Hardly limiting the device access memory to 4 GB for integrated vulkan devices here. This works for the Steam Deck in order not to go above 4 GB VRAM usage any more (above this value the likelihood to crash when the RAM exceeds 12 GB as well raises).

But there will be perhaps a detection mechanism necessary for detecting the real memory limit for integrated vulkan devices. Those likely might have small limits anyway, but what about integrated GPUs on machines with > 16 GB RAM, aka larger amounts ?