Can't generate mipmaps if the format is
not color renderable or not filterable.

Change-Id: I919a4c29c17a69c25e018dd4682940eecd4df30e
Reviewed-on: https://swiftshader-review.googlesource.com/14068Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>