Tweak default values of tunables and use larger default step size

It is more interesting to get smaller text in the initial tile. Also,
make the tile fit on the screen of an iPad.

Change-Id: Ibe4e7f055ec2782a4fd96a3ae585d4630449f0ee
It seems that calling SetSwVisArea() has no effect

Make it optional at run-time (for now while testing) with an
environment variable.

Change-Id: I2ba1673fc51e91d0943148a0bf41351c87f272e5