use RGB texture to save two floats per cell
make the vertex array setup also forward compatible post-3.2
fix overflowing the maximum texture size

We need to split the geometry in blocks of 512k cells so that
we can accomodate for most (all ?) the H/W from the last 5 years
that also support the shader instancing method.
reintroduce proper lighting
remove debugging output
fix call of updateMainNode when no tree has been set yet
move uniform update at the top of updateMainNode for clarity
remove unnecessary drawables in updateMainNode