Enhancement: Multi-sampling

[neurolabusc]

This tutorial was really helpful in helping me port GTK2 OpenGL applications to GTK3. However, the resulting projects do not look as nice, as it is unclear how to enable multisampling with GTK3. This is best seen in high contrast edges, such as white on black. Below on the left is your GTK3 project with the background set to black and the top vertex set to white. Note the jagged appearance of the right triangle. The left is what is generated with GTK2 using x4 multisampling: the aliasing of the edges is much nicer, with partial volume represented by gray. Is there any way to enable multisampling with GTK3 OpenGL?