Fix #107704: Release infrequently used memory in Metal buffer pools

Excessive memory pool bloating could occur for certain workloads in
Metal. Particularly those which continuously allocate increasingly
large buffers with minimal re-use of existing buffers.

New logic added to the memory pool flushes old buffers if they
have not been used for a set period of time. Timing is calibrated
against system resources and overall memory pressure.

Metal memory pressure will run higher than OpenGL, however,
this is an active decision to provide significant performance
improvements for scenarios which allocate lots of memory and
for keeping frames queued in flight without stalling on pending
GPU work.

Authored by Apple: Michael Parkin-White

@@ -278,0 +297,4 @@
+       * the application is using. */
+      time_t deletion_time_threshold_s = 600;
+      /* Spare pool memory >= 2GB. */
+      if (allocations_in_pool_ >= 2147483648LL) {