Render to texture #29, Particle VBO #28

This adds rendering to texture (color and depth buffer), passed to the screen via flush shader.
The buffer resizing does not work in the moment, but for the presentation it should be okay to start the game in full screen mode (see main.cpp).

This also includes particle_vbo_28, because the debugging osg sphere cause program crashes with rendering to texture?! Seem that most osg drawables/shapes don't work with modern OpenGL (vertexAttributeAliasing enabled), at least not on amd.
Please add particle rendering (#28) related comments to #32.