ContextVk's staging buffer never gets a chance to free its free buffer
list. During application load time, a large amount of memory may be
allocated from this buffer to stage texture updates and they would
remain throughout the life of the application.

This change ensures that the free buffer list doesn't grow unbounded. In
the Manhattan trace, this saves >1GB of memory on Linux.

There are now three policies for vk::DynamicBuffer:

- Always reuse buffers: This is useful for dynamic buffers that make
  frequent small allocations, such as default uniforms, driver uniforms,
  default vertex attributes and UBO updates.
- Never reuse buffers: This is for situations where the buffer is
  unlikely to be used after some initial usage, such as texture data
  upload or vertex format emulation (as the conversion result is cached,
  so it's never redone).
- Limited reuse of buffers: For the staging buffer in the context which
  is shared by all immutable texture data uploads, it's useful to keep a
  limited number of buffers (1 in this change) to support future texture
  streaming while allowing a large number of buffers allocated in a
  burst to be discarded.

Bug: angleproject:5690
Change-Id: Ic39ce61e6beb3165dbce4b668e1d3984a2b35986
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2725499
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>