Fix use of Canvas.clipPath with hardware acceleration

[daniele-athome]

After a few months, finally :-) a fix for #100. I've also included - in a separate commit - minSdkVersion downgrade to 15, but that's for my own purposes - and it works anyway, so I don't see a reason for not doing so. You're free to skip that commit of course, but I'll need to keep my fork in case you do.