Initial testing using benchmarks shows that the majority of the
descriptor pools allocate fewer than 32 descriptor sets worth of
descriptors. This CL reduces the initial size of each pool from 128 to
32 to reduce memory consumption.

Additionally, when a pool is exhausted and a new one is created, the
size of the pool doubles each time, up to a max of 512 descriptor sets
worth of descriptors.   This allows us to aggressively increase the size
of the pools that appear to be very hot and decrease the total number of
pools created.

Bug: angleproject:5067
Test: CQ
Change-Id: I190059cf04134902d6251d475dd908c1cbb82b58
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2430193Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>