Fragment output layout qualifiers may contain location,
which was currently ignored by SwiftShader. Pre-declared
fragment outputs that have a valid location at the correct
location within the fragmentOutputs array to solve this.

Fixes all failures in WebGL 2 tests:
all/deqp/functional/gles3/fragmentoutput/*

Change-Id: I30e004449fb90713984b3481abb24c5d0cd6e867
Reviewed-on: https://swiftshader-review.googlesource.com/16788Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>