This is a reland of 4b92e089
Initializes mPadding to 0 in SamplerDesc::update

Original change's description:
> Vulkan: Add support for EXT_texture_border_clamp
> 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/+/2830192
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
> Commit-Queue: Mohan Maiya <m.maiya@samsung.com>

Bug: angleproject:3577
Change-Id: I8684242c4bce6e1a006dbe926defaa495fcc2282
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2911571Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>