Allocate the backbuffer just in time

On my system with a 1440*900 resolution of FB_VISUAL_TRUECOLOR (i.e. 32 bits
per colour), the library allocates a backbuffer of ~5 MiB - probably without
ever using it. This waste is prevented by allocating the backbuffer when it
is first referenced.

The laytest program now uses the framebuffer to draw the whole animation to
the backbuffer. This works but is currently not very efficient because the
whole backbuffer is always written to the screen which is, again, around 5
MiB per animation. We ought to use the optimisation structure...

Nevertheless I suspect that laytest now runs with less flickering.