Add support for GL_EXT_texture_border_clamp. This is implemented by
using VK_EXT_custom_border_color.

Bug: angleproject:3577
Test: dEQP-GLES31.functional.texture.border_clamp*
Change-Id: Ie9fa1eb5dd03b997b5ae182787641a53080a9e51
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2830192Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>