Implement GL_GENERATE_MIPMAP. Bug 21278131 Change-Id: I6252b84f0fd810496475a6e19c9f131bb69d977f Reviewed-on: https://swiftshader-review.googlesource.com/3194 Reviewed-by: Nicolas Capens <capn@google.com> Tested-by: Nicolas Capens <capn@google.com>