[Impeller] Started using a pool for HostBuffers.

[gaaclarke]

This removes ~792 MB/s of allocation when scrolling around the Gallery ((358 MB - 226 MB) * 6).

before

after

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[gaaclarke]

@chinmaygarde I'm guessing this isn't exactly how you want things laid out. Let me know where you want me to put stuff.

[gaaclarke]

@jonahwilliams I made the pool threadsafe and I limited its size to a certain number of bytes (1MB), throwing away any buffer that happens to grow over 512K.

I also briefly looked into trying to convert the RenderPass create functions to unique_ptrs since in practice they don't actually have to be shared_ptr and it makes finding things like being passed to another thread harder to find. The change became difficult since our tests rely on those being shared_ptrs.

[gaaclarke]

I also punted on using a lockless ring buffer for now.

[jonahwilliams]

I think this approach looks good to me. We have a similar pattern for recycling objects but it is vulkan specific, and I belive @chinmaygarde wants to hoist this up so we can use it in a non-backend specific manner.

So lets make sure this implementation can converge with the plans for texture recycling.

[chinmaygarde]

I'm going to pick up the recycling work after subpasses. So not making any progress on it right now. Let's land this if it LGTY and handle the bigger recycling update later.