Corrected initial traversal index

Initial index was wrong when multiple viewports are active.
const correctness fixes
Added i increment.
Fix opengl headers for Mac osX
fix: libc++ is more picky when it comes to const methods
Fix: provide a cast operator to unisgned char to fix libc++ compile issue
fix: Eigen doesn't compile in clang without increasing template depth
Merge pull request #357 from fran6co/fix-opengl

Fix opengl headers for Mac osX
Merge pull request #362 from fran6co/mac-10.9-fixes

[rebase] fixes to make it work on mac os x 10.9 mavericks
Merge pull request #337 from bucienator/master

fixed const correctness
Merge pull request #351 from jakobwilm/visualizer-fixes

Corrected initial traversal index