Remove manual clamping/rounding from ColorSpace and PDFImage, by having their methods use Uint8ClampedArrays

The built-in image decoders are already using `Uint8ClampedArray` when returning data, and this patch simply extends that to the rest of the image/colorspace code.

As far as I can tell, the only reason for using manual clamping/rounding in the first place was because TypedArrays used to be polyfilled (using regular arrays). And trying to polyfill the native clamping/rounding would probably have been had too much overhead, but given that TypedArray support is required in PDF.js version `2.0` that's no longer a concern.

*Please note:* Because of different rounding behaviour, basically `Math.round` in `Uint8ClampedArray` respectively `Math.floor` in the old code, there will be very slight movement in quite a few existing test-cases. However, the changes should be imperceivable to the naked eye, given that the absolute difference is *at most* `1` for each RGB component when comparing `master` and this patch (see also the updated expectation values in the unit-tests).

src/core/evaluator.js

@@ -377,7 +377,8 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
         var width = dict.get('Width', 'W');
         var height = dict.get('Height', 'H');
         var bitStrideLength = (width + 7) >> 3;
-        var imgArray = image.getBytes(bitStrideLength * height);
+        var imgArray = image.getBytes(bitStrideLength * height,
+                                      /* forceClamped = */ true);
         var decode = dict.getArray('Decode', 'D');
 
         imgData = PDFImage.createMask({