fix webgl parameter

[CodingFabian]

I just spent 20 minutes figuring out why my webgl doesn't work.
First I tried "disableWebGL=false" because in api.js this is the name of the property and it is default initialized with true.
Then i found that this value is actually overwritten by reading from Preferences.
in preferences a value "enableWebGL" is stored. once initialized the value is negated and assigned to disableWebGL.
So I tried using "enableWebGL=true". This also did not work.
Then i figured out that the Preferences are prefilled with a value read from the URL. the value read from the url is named "webgl"

Now finally after setting "webgl=true" the preference "enableWebGL" is set to true, which on init, will set API.js PDFJS.disableWebGL, which by default is inited to true, to not the value of enableWebGL, so any check for WebGLUtils.isEnabled will no longer bail out on PDFJS.disableWebGL.

I hope you get the point :-)

[yurydelendik]

We are trying to rely less on the hash parameters to configure the viewer: most of the parameters are moved to the preferences. In the future we probably disable these configuration parameters if user did not enable the pdfBug.