clients: Add a mandelbrot set shader to simple-dmabuf-egl

Support drawing a mandelbrot set in the fragment shader, rendering it
with separate draw calls, one for each cell in a virtual 4x4 grid. This
more complex and heavy drawing will potentially help us to visually
discover any present or future explicit synchronization issues.

The mandelbrot set rendering is enabled with the -m/--mandelbrot
command-line switch.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>