Fix circular imports

```
$ npm run build
[...]
┌──────────────────────────────┬──────────┐
│ File name │ Size │
│ ---------------------------- │ -------- │
│ image-display-control.min.js │ 17.88 kB │
│ ---------------------------- │ -------- │
│ Totals │ 17.88 kB │
└──────────────────────────────┴──────────┘
(!) Circular dependencies
dist/src/PositionInRelativeCoord.js -> dist/src/PositionInPixels.js -> dist/src/PositionInRelativeCoord.js
dist/src/SizeInPixels.js -> dist/src/SizeInRelativeCoord.js -> dist/src/SizeInPixels.js
created ./dist/image-display-control.min.js in 546ms
```

Run `npm run build` and inspect the reported dependency cycles between dist/src/PositionInRelativeCoord.js and PositionInPixels.js, and between dist/src/SizeInPixels.js and SizeInRelativeCoord.js. Trace these cycles back to their TypeScript sources and remove the circular imports; done means the build completes without either circular-dependency warning.

Written by the indexing model from the issue text.