Set default tile-size to 128x128

Performance of different tile-sizes on one given system, the tests are the
tests in the perf subdir. All numbers are megabytes/second, the gimp
projection test is for 8bpc gamma corrected on a 3 layer 8000x4000 document.
best result(s) marked with *.

                  128x128 256x64 64x128 128x64 64x64 32x32 512x64 256x256 16x16
gaussian-blur:      138*   128    139*   135   130    124   101     99      43
bcontrast:          646    653*   623    640   500    180   384    375     413
unsharp-mask:        87*    82     87*    86    87*    78    58     53      46
rotate:              32     35     11     33    28     76*   37     35      70
rotate-nearest:      38     39     12     38    31    115*   44     41     102
bcontrast_4x:       445*   432    442    344   415    381   102    303     295
bcontrast-minichunk:207    204    229    227   237    249*   52     12     213
gimp projection:    112    104     83     90    61     21   130*    88       3