ui: fonts are scaled down

With the default settings, I have very tiny font size on windows.
It's about 6 pixels height (for the `defaultFontSize` of 11).

Shiny's size event reports 1 PixelPerPt.
main.go creates a Win with `dpi: float32(e.PixelsPerPt) * 72.0`

truetype.NewFace also scales with 72: `scale: fixed.Int26_6(0.5 + (opts.size() * opts.dpi() * 64 / 72))`

Why does `ui.NewWin(dpi float32)` call NewFace with a DPI scaled down by 72/96?
```
face := truetype.NewFace(defaultFont, &truetype.Options{
Size: float64(defaultFontSize),
DPI: float64(dpi * (72.0 / 96.0)),
})
```

Start in main.go at the Win creation and follow the dpi value into ui.NewWin and its truetype.NewFace call. Compare Shiny's PixelsPerPt with the DPI passed to the font options; done means the default Windows font is no longer rendered at the reported tiny size.

Written by the indexing model from the issue text.