Use global indices array to draw layers

An array of indices local to a layer would only be necessary if
we changed the way we resolve T-junctions. Since we only ever
draw quads, let's just use the indices we use everywhere else.

This change also uses the global indices array to render list
of colored rectangles to save on the number of vertices generated
CPU-side.

Change-Id: Ia6d1970b0e9247805af5a114ca2a84b5d0b7c282