Addition of the entry points for GL_KHR_blend_equation_advanced
extension.

Bug: angleproject:3586
Change-Id: I7a651c19a4b3d0a7cfe4af7813f1530d27704bc1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2830140
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>