shiny/example/tile: don't hard-code the font face ascent.
shiny/example/tile: don't hard-code the font face ascent.

Change-Id: Ib40a79886da7fcfe41301d3811f14c53b579bc61
Reviewed-on: https://go-review.googlesource.com/21067
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

AkihiroSuda/golang-exp-aspectgo,golang/exp,AkihiroSuda/golang-exp-aspectgo,golang/exp,golang/exp,AkihiroSuda/golang-exp-aspectgo