VideoBackends:Vulkan: Allocate descriptor pools as needed

[K0bin]

Dolphin currently creates one massive descriptor pool per frame. If it ever runs out of descriptors or descriptor sets,
it executes the current command buffer, hoping that will result in a fresh pool.

The problem is that after #11090, executing the current command buffer will not result in a fresh descriptor pool.
That PR decoupled command buffers and descriptor pools to allow us to use more command buffers which in turn reduces
potential waiting for the previous one to finish when flushing more often (mostly on weak mobile GPUs).

So right now, when descriptor set allocation fails, it just starts a new command buffer and then tries again with the exact same descriptor pool. I don't know how often this actually happens in games, given that the descriptor pool is pretty huge but we should still fix this.

So instead of just using 1 fixed pool per frame, I now create them as needed. I also picked way smaller descriptor and set counts because it will just create as many sets as necessary.
This should also reduce memory usage ever so slightly but that depends on the Vulkan driver and I don't think it's significant so I haven't measured it.

[TellowKrinkle]

I don't know how often this actually happens in games, given that the descriptor pool is pretty huge

F-Zero GX runs out of SSBO descriptors (there was only 16 per pool, and it needs two for every command buffer flush) on MoltenVK (I think other drivers don't count each type separately, but MoltenVK does), which this fixes.

[Testsr]

Any progress on testing this?

[AdmiralCurtiss]

Can someone with Vulkan knowledge do a review on this?

[iwubcode]

Tested a handful of Wii titles. Code LGTM.