Make layer distance changable. Refs #1.

As a side-effet, now the ordering of the layers is left-to-right rather
than right-to-left. This shouldn't matter in normal processing, but it
is interesting nonetheless.

src/fmri/RenderingState.cpp

@@ -236,7 +236,7 @@ void RenderingState::render(float time) const
 
 
     glPushMatrix();
-    glTranslatef(5 * currentData->size(), 0, 0);
+    glTranslatef(-LAYER_X_OFFSET / 2 * currentData->size(), 0, 0);
 
     for (auto i : Range(currentData->size())) {
         glPushMatrix();