Attempting to fix the "black bars" problem seen on Macs.  Noticed that the Mac assigns vertex attributes in a different order from other platforms, so assuming that "attribute vec3 position" is the same attribute ID in both vertex shaders is no longer a valid assumption.  Added code to track the attribute IDs separately.
Attempting to fix the "black bars" problem seen on Macs.  Noticed that the Mac assigns vertex attributes in a different order from other platforms, so assuming that "attribute vec3 position" is the same attribute ID in both vertex shaders is no longer a valid assumption.  Added code to track the attribute IDs separately.

QingchaoHu/glsl-sandbox,RobertoMalatesta/glsl-sandbox,CortesHub/glsl-sandbox,beni55/glsl-sandbox,oresmus/glsl-sandbox,QingchaoHu/glsl-sandbox,mrdoob/glsl-sandbox,CortesHub/glsl-sandbox,CortesHub/glsl-sandbox,mrdoob/glsl-sandbox,oresmus/glsl-sandbox,mrdoob/glsl-sandbox,RobertoMalatesta/glsl-sandbox,mrdoob/glsl-sandbox,beni55/glsl-sandbox,oresmus/glsl-sandbox,beni55/glsl-sandbox,QingchaoHu/glsl-sandbox,RobertoMalatesta/glsl-sandbox