Don't reassign the canvas context font if there is no change.
Fix #718 to prevent a sub class to reinitialize the static members of its super class.
Merge pull request #720 from yxl/static_init

Fix #718 Prevent <clinit> of the super class being called more than once
Merge pull request #721 from yxl/withFont

Don't reassign the canvas context font if there is no change.