Handles the ES 3.1 relative offset parameter in vertex attributes by
adding it to the binding offset.

Test: ./angle_deqp_gles31_no_gtest --deqp-egl-display-type=angle-vulkan -n dEQP-GLES31.functional.vertex_attribute_binding.usage.single_binding.unaligned_offset_elements_1_aligned_elements
Bug: angleproject:3598
Change-Id: Idbbd5ba4868a4dfc8f99188a84a5cd1374e09065
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1724453Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: James Dong <dongja@google.com>