Cube images currently have a 1 pixel border around them for seamless
cubemaps. To add a border around a compressed image would require
adding an entire block's width/height around the image at each mip
level, which would take a lot of extra space and wouldn't be so
useful right now, since we'll perform both image decompression and
cubemap border update before sampling, so might as well do both at
the same time and only store a border in the decompressed image,
so that compressed cube images don't need to keep a border.

Bug b/119620767

Tests: dEQP-VK.pipeline.sampler.view_type.cube.format.*

Change-Id: I17974c0148fded37bca7c17a78d08d5e683a2afb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28548Tested-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>