suggestion: configuration parameter for VRAM usage

[nihui]

suggestion: configuration parameter for VRAM usage

[teamolhuang]

update: seems like it always exit out when it's at first gap sync gap3.

tested on 20220318 version so there is logging.

I've also find a weird behaviour when testing with waifu2x-ncnn-vulkan. On default setting for 4x same issue happens, but when I raised threads to like 16:16:16 it can run and complete things. It suprised me because usually I'd think less threads will be stabler??

Sadly the same trick didn't work on realcugan-ncnn-vulkan, though higher threads does show vkAllocateMemory failed -10. Through AMD panel, on something like 8:8:8, I saw VRAM being consumed to 8GB, but then it just exited. on 1:1:1 it consumed to ~1.3GB and then the program exit without any message.

But for now at least CPU mode is working for me in a reasonable time. Still looking for this issue's solution for GPU though.