The default alpha value for integer types is 1 and not the
largest representable value for that type, as it was
previously implemented. A value of 1 still means that the
texture is opaque. This fixes ~60 fragment output tests.

Change-Id: I96650fc8ba4812dcb8a2787979c6829f14f0960e
Reviewed-on: https://swiftshader-review.googlesource.com/7190Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>