Addition of support for immutable storage to buffer objects. Also adds new end2end tests for these usecases * Basic BufferStorage * SubData update * map/unmap buffer Bug: angleproject:5056 Tests: angle_end2end_tests --gtest_filter=BufferStorageTestES3*Vulkan Change-Id: Iba74b372ad033711927b63c6a04cec0eeb4db699 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2419952Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
