Underflow in restore with PGraphicsAndroid2D

[codeanticode]

The use of canvas.save()/restore() sometimes causes the following exception:

java.lang.IllegalStateException: Underflow in restore - more restores than saves
at android.graphics.Canvas.native_restore(Native Method)

particularly when using push and popMatrix(). This issue has been reported elsewhere:

http://stackoverflow.com/questions/23893813/canvas-restore-causing-underflow-exception-in-very-rare-cases
traex/RippleEffect#76

and seems to appear in Android 6.