VV_GLERROR macro

Produces a gl error string including __FILE__ and __LINE__
Split offscreenbuffer::unbind() function

unbind() and blit() are separate operations
Redesigned preferences dialog

Use a tab widget instead of a vertically stacked layout for
better overview
Add support for interlaced stereo modes
Add support for side-by-side stereo mode
Add support for red-cyan anaglyph stereo mode
Choose fbo alpha compositing for texrend
GUI to adjust ocular eye distance
Merge branch 'dev' of https://github.com/deskvox/deskvox into dev

Conflicts:
	vox-desk-qt/src/vvcanvas.cpp