implement clipToDocumentBounds option to getPixmap

Adds an optional (`false` by default) property to getPixmap's `settings` parameter called `clipToDocumentBounds`. If set to `true`, the resulting thumbnail is clipped to the document bounds. (Otherwise, in the default case, all pixels are returned, including those that might lie outside of the document bounds.)

**Important:** This will not work if an inputRect/outputRect scaling is specified. In that case, this property is ignored.

An easy way to test this is to comment out the inputRect/outputRect lines at the beginning of `getPixmap` (where `params` is constructed), and then set `params.clipToDocumentBounds` to `true`.

Addresses adobe-photoshop/generator-assets#255 and partially addresses adobe-photoshop/generator-assets#235

cc @nimbupani @jhatwich