libgui|GL: Perform multisampling with renderbuffers

With OpenGL 2.1, the only way to render into a texture with
multisampling is to use OpenGL extensions.

GLFramebuffer now supports specifying a sample count. If more than
one sample is requested, a secondary target is created with
multisampled renderbuffers. Copying the multisampled contents to
the attached texture occurs automatically.

Todo: Debug Oculus Rift’s multisampled framebuffer (one side visible
only).