Use correct alpha range for the distance field outlined text.

Use correct alpha range for the distance field outlined text.

The alpha treshold values used in the outline shader were hard-coded. It
should use the same logic as in the normal text shader, using the
ThresholdFunc and the AntialiasingSpreadFunc for calculating these
values.

Change-Id: I0e6f38644692cc7a8a4cee1e8e60622fe289d615
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>