There are only 241 unique format enums at this moment, so they can fit
in an 8-bit variable instead of the 32-bit enum.

Bug: b/148016460
Change-Id: I00d47a87ac0d053aee44a5d18f3c96d78c0f7cb0
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42528
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>