extend getPixmap API to allow specifying layer index ranges

The easiest way to test this is to create a document with (for example) three layers and then shim in the line: `layerSpec = {firstLayerIndex: 0, lastLayerIndex: 2};` at the very beginning of the `getPixmap` implementation.

Then, just fire up the generator-assets plugin, tag any ol' layer with an asset tag, and you should get an asset with all the layers.