This PR contains changes for BodyPix.js 2.0 release.

* Adds ResNet (outputStride=32 and 16 are supported) to BodyPix library.
* Adds multi-person pose, segmentation, and body part support.
* Switches to use GraphModel.
* Switches the MobileNet from LayerModel to GraphModel.
* Adds configuration validation.
* Updates person & body part segmentation decoding logic.
* Updates the camera demo UI to support switching between MobileNet and ResNet.
* Image/video are padded and resized to a square shape using the closes accepted size.
* `estimateSegmentation` methods performs semantic segmentation (and single-person pose).
* `estimateMultiPersonSegmentationByInstance` performs instance segmentation (and multi-person pose).