[#608] added a private `globalColor' object in the canvas renderer to cache the current global color

this properly fixes the issue with cocoonjs/canvas+ where the
`fillStyle` property is undefined, by using this `globalColor` property
for the `setColor` function.
this also matches the webGL renderer implementation !
[#608] added parameter check for both parseRGB and parseHex and not throw an exception if the given arguments are not valid