Managing opacity properties on polygon and path object

Adding:
ctx.globalAlpha = this.group ? (ctx.globalAlpha * this.opacity) : this.opacity;
after ctx.beginPath(); on path and polygon class.
it fixes rendering on this svg from actual kitchensink

```xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" width="512px" height="512px" viewBox="0 0 512 512" enable-background="new 0 0 512 512" xml:space="preserve">
<g>
	<g>
		<path fill="#231F20" d="M119.387,20.312h21.298v21.045h19.485V20.312h21.303v63.727H160.17V62.7h-19.485v21.338h-21.298V20.312z"/>
		<path fill="#231F20" d="M209.482,41.444h-18.754V20.312h58.812v21.133h-18.759v42.594h-21.3V41.444z"/>
		<path fill="#231F20" d="M258.879,20.312h22.21l13.661,22.392l13.648-22.392h22.219v63.727h-21.212V52.453L294.75,75.111h-0.366    l-14.665-22.658v31.585h-20.84V20.312z"/>
		<path fill="#231F20" d="M341.219,20.312h21.308v42.664h29.955v21.062h-51.263V20.312z"/>
	</g>
	<path fill="#231F20" d="M200.662,266.676H256v-42.92h-59.169L200.662,266.676z M88.686,111.982l30.47,341.74l136.762,37.966   l136.891-37.948l30.507-341.758H88.686z M366.694,431.981L256,462.668v-43.494l-0.067,0.02l-85.858-23.835l-6.004-67.298h42.075   l3.116,34.914l46.68,12.607l0.059-0.019V308.59h-93.669l-11.306-126.749H256v-41.914h136.766L366.694,431.981z"/>
	<path opacity="0.8" fill="#231F20" d="M307.592,308.59H256v66.974l46.728-12.613L307.592,308.59z M256,139.927v41.914h104.975   l-3.754,41.915H256v42.92h97.406l-11.499,128.683L256,419.174v43.494l110.694-30.687l26.071-292.055H256z"/>
	<g opacity="0.2">
		<polygon fill="#231F20" points="256,181.841 151.025,181.841 162.331,308.59 256,308.59 256,266.676 200.662,266.676     196.831,223.756 256,223.756   "/>
		<polygon fill="#231F20" points="256,375.563 255.941,375.582 209.262,362.975 206.146,328.061 164.07,328.061 170.074,395.358     255.933,419.193 256,419.174   "/>
	</g>
</g>
</svg>
```

![image](https://cloud.githubusercontent.com/assets/1194048/3325437/d4f0cfe0-f78e-11e3-9602-4c8092452261.png)

![image](https://cloud.githubusercontent.com/assets/1194048/3325458/196238bc-f78f-11e3-8b21-72594be7f757.png)