`e.g.` looks nicer than `eg`

Adding many more `eg` to `e.g.` replacements.

I ran:

```bash
 find agg24/ CXX CXX5 doc examples/ lib/ LICENSE/ release/ src/ test/ ttconv/ unit/ -type f -exec sed -i 's/\beg\b/e.g./g' {} \;
```

and then manually filtered for errors.