Shadow buffers help reduce the latency of glMap* operations
at the cost of CPU overhead. It might not be desirable to
incur such an overhead for all usecases. Featurize it but
enable it by default.

Bug: angleproject:4339
Change-Id: I6374618bf99677eef55fd50a139fb86f5ea70791
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2278102Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>