Direct imageData doesn't consider devicePixelRatio

[anderoonies]

Describe the bug
When directly extracting getImageData from the canvas, the devicePixelRatio calculations aren't considered and the result is improperly sized.

To Reproduce
https://codehs.com/sandbox/andy/device-pixel-ratio

Steps to reproduce the behavior:

1. On a device with >1 devicePixelRatio, extract imagedata using __graphics__.getContext().getImageData()
2. Create a new WebImage and set its image data to the extracted data with setImageData
3. The WebImage only contains a 1/n * 1/n portion of the ImageData, where n is the devicepixelratio

Expected behavior
I'd like getImageData to only ever return a value the size of the perceived canvas (after css downscaling).
I think a method like getImageData() (not on context, just global) that performs the devicePixelRatio scaling would be helpful. This is advanced functionality, but something to think about.