Resize DimLayer explicitly on rotation.

Using the alpha value to trigger a resizing of the DimLayer was a
Bad Idea. The alpha value should reflect the true alpha value and
not be used to trick the code. Actually changing the size and
position is a Better Idea.

Fixes bug 13101776.

Change-Id: I11c16b8276919ea85960fe87bb17c0956ce8a3b1