By using a C array instead of a std::vector, the data tables for BC6h
are initialized without a constructor. This helps lower SwiftShader's
startup costs.

Tests: dEQP-VK.*bc6*
Bug: b/174782098
Change-Id: I9bcfa2753bc6255ff16543d424cf8936c2817148
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/54788Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Sean Risser <srisser@google.com>
Commit-Queue: Nicolas Capens <nicolascapens@google.com>