Use correct pixel ratio when creating image buffer from map.addImage()

[asheemmamoowala]

This closes #4637

In addition to our code not computing the pixelRatio of the passed in image for display correctly, I found that the Sprite Atlas class was not setting its internal pixelRatio to match the device pixel ratio during construction. This should fix both issues.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
  - [ ] document any changes to public APIs
  - [ ] post benchmark scores
• manually test the debug page

[mourner]

Just wondering, do we have a way to emulate retina on the render tests to catch issues like this?

[kkaefer]

Just wondering, do we have a way to emulate retina on the render tests to catch issues like this?

Yeah, you can specify "pixelRatio": 2 in the metadata section of the render test. There are already a few existing tests for retina. It'd be cool to have a test for this, and check GL native for the same bug.