Improve color clear precision.

Change-Id: Ib9dadf3d8fee0f63deb10e6754856c0530c928ab
Reviewed-on: https://swiftshader-review.googlesource.com/3995
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>