BUG: Invalid read for even sized windows

If a window had a row size that was even, the stereo split viewport
horizontal code tried to copy beyond the bounds of the
window. valgrind reported "Invalid read of size 1" for each component.

This patch adjusts the mid point if the window sife is even.

Change-Id: Ibbe7bfd7c06df6d67335092248d3ffd70f812cd5
Merge topic 'InvalidRead_RenderWindow' into master

92f43d26 BUG: Invalid read for even sized windows