The Display now owns scratch buffers and loans them out to contexts
while they are current.  This allows us to to only allocate one scratch
buffer in a single-threaded use case.

Tick the scratch buffers every time a new context is made current.

Lower the lifetime from 1000 to 64 to ensure that in the worst case,
the buffers are cleared after not being used for ~1 second.

BUG=chromium:1030835
BUG=angleproject:4363

Change-Id: I83552424e2beac62b9e41152876b04fc84f53692
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2031698
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>