Also contains the following changes:
* Replace the use of `unsigned char` with `uint8_t`. `char` is not guaranteed to be 8-bit.
* Remove the `dstW` and `dstH` parameters from `BC_Decoder::Decode` and `ETC_Decoder::Decode`. They're always the same as the `w` and `h` parameters.
* Add BC6 types to various switch cases. The actual decode logic is not implemented for these formats.

Tests: *bc7*
Bug: b/151203718
Change-Id: I7b232b9dc3a9b02d172f87a62c88ce56b2cca956
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41508Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>