Added a block compression (BCn) decoder and logic for all
formats for n in the [1, 5] range.

The decompression follows the same logic as the ETC2
decompression in vk::Image.

Tests: dEQP-VK.*bc*
Bug: b/146052572
Change-Id: I64fac0a7af52e1be209c1cfd5373744918c7df14
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39369
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>