Fill the initial JBIG2 buffer with 0xFF only if defaultPixelValue is set

Per the TypedArray specification, ArrayBuffers are initialized with zeros.

http://www.khronos.org/registry/typedarray/specs/latest/#5
Skip the Math.sqrt for clamped values in convertToRgb

No need to compute the square root of clamped values.
Only the values in the range ]0,1[ need to be processed.
Made viewer's webViewerLoad callable from outside viewer.js

To allow manual triggering when DOMContentLoaded can not be relied upon.
Merge pull request #4345 from p01/patch-1

Skip the Math.sqrt for clamped values in convertToRgb
Merge pull request #4346 from p01/patch-2

Fill the initial JBIG2 buffer with 0xFF only if defaultPixelValue is set
Merge pull request #4333 from dferer/requirable-viewer

Made viewer's webViewerLoad callable from outside viewer.js