Fixed glsl compilation error on OS X 10.9

Renamed 'step' variable, which seems to conflict
with built-in step function, resulting in compilation
error on RogueResearch6 dashboard (OS X 10.9).

Change-Id: Ibee90a9ef3e86d9731103c01d04c23e154f70d07
Merge topic 'glsl-reserved' into master

1ce7c91d Fixed glsl compilation error on OS X 10.9