glbench: add buffer streaming tests

native: buffer_streaming_subdata           = 5.53 mbytes_sec
        buffer_streaming_subdata_overwrite = 5.65 mbytes_sec

virgl:  buffer_streaming_subdata           = 4.95 mbytes_sec
        buffer_streaming_subdata_overwrite = 0.01 mbytes_sec

BUG=chromium:959432
TEST=glbench -notemp -tests buffer_streaming

Change-Id: I84ef8bc36aa2fce2d255bd14838318d0d2eeb2c3
Reviewed-on: https://chromium-review.googlesource.com/1595059
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Chia-I Wu <olv@google.com>
Reviewed-by: Po-Hsien Wang <pwang@chromium.org>
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>
4 files changed