fixed scene offscreen export

fixed GL state duplication for offscreen rendering context
added several checkpoints for compile- and run-time OpenGL compatibility
added color blend mode to tube and hsc rendering
updated to newest glew version
fixed py exports for GfxObj and GfxObjBase
fixed py shortcuts for line-trace and trace render options



git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/branches/new_gfx@2285 5a81b35b-ba03-0410-adc8-b2c5c5119f08