Textures with just one texel don't require filtering. This also avoids precision
issues for pedantic tests which expect exact values.

Bug swiftshader:39

Change-Id: I52c5697e277523e2b8cb2c879ba0a7b2296ee7c5
Reviewed-on: https://swiftshader-review.googlesource.com/9068Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>