This tests different DrawElements*BaseVertex draw calls
from different extensions including OES_draw_elements_base_vertex,
EXT_draw_elements_base_vertex, and ANGLE_base_vertex_base_instance,
with various combinations of base vertex, base index values.

Bug: angleproject:4536
Change-Id: I3cd256522684c6040199d7704aac8575237dbd96
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2146292Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shrek Shao <shrekshao@google.com>
Commit-Queue: Shrek Shao <shrekshao@google.com>