Fix Context2D to clear the current shader before drawing.

Context2D uses default shaders for some of its drawing. Minor
change to clear the cuyrrent shader back to a default one.

Change-Id: I7e4e47371713ce360adf5e0fa53c018c76188baf
Merge topic 'context_shader_issue' into master

18202904 Fix Context2D to clear the current shader before drawing.