This is so that the resulting buffers can be written to by the GPU.

Additionally, the class is given the ability to create host-visible or
device-local buffers, making map()-on-init() optional.

This is in preparation for vertex/index transformations in compute.

Bug: angleproject:2958
Change-Id: Ib8f5829e33a1e49fa8f80c70dbde74f313ae49ec
Reviewed-on: https://chromium-review.googlesource.com/c/1351113
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>