Added mix implementation with a bvec as a 3rd argument,
which is basically a select.

Fixes 18 failures in dEQP-GLES3.

Change-Id: Ifaf4a27e1a25fbaad979a7d26ad4a424631acd08
Reviewed-on: https://swiftshader-review.googlesource.com/16288Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>