Shifts the (normalized) viewport into x-direction, i.e. a value of 0.5
lets the rendering start in the middle of the framebuffer so that only
the leftmost half of the image is visible in the right half of the
framebuffer.
Shifts the (normalized) viewport into y-direction, i.e. a value of 0.5
lets the rendering start in the middle of the Framebuffer so that only
the uppermost half of the image is visible in the bottom of the
framebuffer.