Optimization!

By using a quadtree optimization, the rendering times can now be reduced
very much.

tfc@graviton mandelbrot 请 cat classic.txt
size       1024 csize        256 its      10000 time         0.339 s
size       2048 csize        256 its      10000 time         0.905 s
size       4096 csize        256 its      10000 time         2.417 s
size       8192 csize        256 its      10000 time         5.905 s
size      16384 csize        256 its      10000 time        18.783 s
size      32768 csize        256 its      10000 time        80.118 s
size      65536 csize        256 its      10000 time       322.732 s
tfc@graviton mandelbrot 请 cat quadtree_optimization.txt
size       1024 csize        256 its      10000 time         0.347 s
size       2048 csize        256 its      10000 time         0.882 s
size       4096 csize        256 its      10000 time         2.293 s
size       8192 csize        256 its      10000 time         4.503 s
size      16384 csize        256 its      10000 time        11.313 s
size      32768 csize        256 its      10000 time        35.975 s
size      65536 csize        256 its      10000 time       147.690 s