vulkan: Add upload fastpath

vulkan: Add upload fastpath

If the memory heap that the GPU uses allows CPU access
(which is the case on basically every integrated GPU, including phones),
we can avoid a staging buffer and write directly into the image memory.

Check for this case and do that automatically.

Unfortunately we need to change the image format we use from
VK_IMAGE_TILING_OPTIMAL to VK_IMAGE_TILING_LINEAR, I haven't found a way
around that yet.