Sanitize display list properties

The comparisons used in the various properties setters could fail badly
in some specific conditions. The scale properties in particular did not
use the same comparisons.

This change also clamps alpha to the 0..1 range which avoids overflow
issues with lowp registers in GLSL computations.

Change-Id: I3e73b584e907a14e2c33d0865ca0d2d4d5bff31d