Fix undefined behavior packing top and left

Shifting a negative signed integer is undefined behavior in
C++11. Convert to unsigned int before packing the word together.

Change-Id: I88251eb753da1bf8887b7f06b3356e828968103a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/657996
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>