Description of the new feature/enhancement

This might be related to #2370, but it addresses a slightly different problem.

Currently, Windows Terminal scales the background image according to your system UI scale. So, for example, if your UI scale is set to 150% in Windows Settings, and you set a 100×100-pixel background image with backgroundImageStretchMode: none, the image will be rendered at 150×150. This might not be what you want, since the scaling will blur sharp edges, and the image will be much larger than you might have expected.

Proposed technical implementation details (optional)

There should be some way to indicate that we want our background image to render pixel-for-pixel, rather than be enlarged according to the UI scale.

Or else maybe it should simply render pixel-for-pixel by default. This was my expectation; not sure if it matches the expectation of others.