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/display/api.js

@@ -671,7 +671,7 @@ var PDFDocumentProxy = (function PDFDocumentProxyClosure() {
      *   title: string,
      *   bold: boolean,
      *   italic: boolean,
-     *   color: rgb Uint8Array,
+     *   color: rgb Uint8ClampedArray,
      *   dest: dest obj,
      *   url: string,
      *   items: array of more items like this