ANGLE uses image blit to generate mipmaps so, for cubemaps,
a border update is required after we blit into an image so
that the cube's border are still seamless when using lower
mip levels.

Fixes WebGL Conformance Test:
conformance/textures/misc/texture-size.html

Bug: b/154620294
Change-Id: I22daf60afd9143c52a57c67ae0039eb49be4f1ca
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44990
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>