Disable default blending when drawing color rect

When drawing a color rectangle (such as drawing the windows background),
the opengl blending mode is always turned on no matter the layer is opaque
or not. This consumes gpu power and memory bandwidth unnecessarily.
The bug was introduced from a refactor of drawColorRect function -
70ca14e0

Change-Id: Ia8996ae0abc1ee9a435d0dc3445809dd45f92b1c