Fix performance issue in Launcher

Bug #3515248

The problem is caused by the fast path when compositing layers on screen.
The fast path draws a single quad using glDrawArrays() whereas the general
path draws an arbitrary mesh using glDrawElements(). It looks like there's
an issue in the driver since glDrawArrays() is significantly slower than
glDrawElements() for a quad (6 vertices!)

This change just gets rid of the fast path.

Change-Id: Ib2361253ec67f44a988270f76c183422f12ce537