Implements ES 3.0 feature GL_PRIMITIVE_RESTART_FIXED_INDEX,
which allows the application to use a fixed "restart" index
to restart the primitive during a single draw call.

This change does't handle GL_LINE_LOOP primitives,
which requires a bit of special handling.

Bug: angleproject:3215
Change-Id: I2388852683fd17328a6a76c48d70a24d67ce8b67
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1650301Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>