For both VK_FORMAT_ETC2_R8G8B8_UNORM_BLOCK and
VK_FORMAT_ETC2_R8G8B8_SRGB_BLOCK, the internal
representation was a 24 bit BGR8 format, which
isn't supported by the texture sampling code,
not is it intended to add support for these
formats. These BGR8 formats were changed to
BGRA8 formats, with the A8 value cleared to
0xFF.

Also fixed 3D compressed textures by taking
depth into account.

Tests: dEQP-VK.pipeline.image.*etc2_r8g8b8_*

Bug b/119620767

Change-Id: I9dd5c34519c9270580170c70b82c2e62fdbba3da
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30709
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>