Infinite loop when photon reflecting from a corner when hitting very close to the edge

[fangq]

The script in #103 also revealed another bug that mirror-boundary condition may hang the GPU when large number of photons are almost exactly hitting the edge. Use the below script to reproduce

clear cfg
cfg.nphoton=1e7;
cfg.vol=uint8(ones(60,60,60));
cfg.srctype='pattern';
cfg.srcpos=[-40,-40, 0];
cfg.srcparam1=[80 0 0 100];
cfg.srcparam2=[0 80 0 100];
cfg.srcdir=[0 0 1 30];
cfg.issrcfrom0=1;
cfg.srcpattern=zeros(100,100);
cfg.srcpattern(51:end,51:end)=1;
cfg.gpuid=1;
cfg.autopilot=1;
cfg.prop=[0 0 1 1;0.005 0.0 0 1];
cfg.tstart=0;
cfg.tend=5e-9;
cfg.tstep=5e-09;
cfg.bc='mm____';

flux=mcxlab(cfg);
mcxplotvol(log10(double(flux.data)))

this could be caused by the limited precision when handling edge-hitting photons.