render/vulkan: increase next descriptor pool size

This ensures that the pool sizes grow exponentially, making the number of pools needed logarithmic in the number of descriptors, instead of linear. Since the first pool's size is 256, this change only has an effect when the compositor creates a large number of textures.

This was originally part of !3869 (merged), but I've split it out since it's a minor change and may be more generally useful.