Optimize PNG compression in SVG backend on Node.js

Use the environment's zlib implementation if available to get
reasonably-sized SVG files when an XObject image is converted to PNG.
The generated PNG is not optimal because we do not use a PNG predictor.
Futher, when our SVG backend is run in a browser, the generated PNG
images will still be unnecessarily large (though the use of blob:-URLs
when available should reduce the impact on memory usage). If we want to
optimize PNG images in browsers too, we can either try to use a DEFLATE
library such as pako, or re-use our XObject image painting logic in
src/display/canvas.js. This potential improvement is not implemented by
this commit

Tested with:

- Node.js 8.1.3 (uses zlib)
- Node.js 0.11.12 (uses zlib)
- Node.js 0.10.48 (falls back to inferior existing implementation).
- Chrome 59.0.3071.86
- Firefox 54.0

Tests:

Unit test on Node.js:

```
$ gulp lib
$ JASMINE_CONFIG_PATH=test/unit/clitests.json node ./node_modules/.bin/jasmine --filter=SVG
```

Unit test in browser: Run `gulp server` and open
http://localhost:8888/test/unit/unit_test.html?spec=SVGGraphics

To verify that the patch works as desired,

```
$ node examples/node/pdf2svg.js test/pdfs/xobject-image.pdf
$ du -b svgdump/xobject-image-1.svg
 # ^ Calculates the file size. Confirm that the size is small
 #   (784 instead of 80664 bytes).
```

test/unit/jasmine-boot.js

@@ -50,6 +50,7 @@ function initializePDFJS(callback) {
     'pdfjs-test/unit/cmap_spec',
     'pdfjs-test/unit/crypto_spec',
     'pdfjs-test/unit/custom_spec',
+    'pdfjs-test/unit/display_svg_spec',
     'pdfjs-test/unit/document_spec',
     'pdfjs-test/unit/dom_utils_spec',
     'pdfjs-test/unit/evaluator_spec',