This implementation utilizes vulkan's dualSrcBlend
feature. Expose this extension if the underlying vulkan
backend allows the use of this feature.

Test: angle_end2end_tests --gtest_filter=EXTBlendFuncExtendedDrawTest*
Bug: angleproject:5074
Change-Id: I7d2f611df89d65e5cac35158cb5f41a0ebd58aae
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2593151
Commit-Queue: Brandon Schade <b.schade@samsung.com>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>