Fix clipping for excanvas and FlashCanvas.
Fix clipping for excanvas and FlashCanvas.

For FlashCanvas, it should just work now :-)
For excanvas we have to make a compromise.  We'll skip clipping on the
overlay context, and allow hits to overflow.  Not fixing that one.

For #118 and #103.

HumbleSoftware/Flotr2,nicgallardo/Flotr2,beastlike/Flotr2,RobertoMalatesta/Flotr2,alanponce/Flotr2,alanponce/Flotr2,jessiejea/Flotr2,alanponce/Flotr2,beastlike/Flotr2,beastlike/Flotr2,jessiejea/Flotr2,nicgallardo/Flotr2,jessiejea/Flotr2,RobertoMalatesta/Flotr2,HumbleSoftware/Flotr2