picopng update + runtime error fix

[alur]

LiteStep is currently using a version of picopng dated 20080503. The latest version is dated 20101224. LiteStep should probably be updated to use the latest version.

On a very related note, the unFilterScanline function in picopng will throw a lot of runtime exceptions when trying to debug LiteStep.

This can be fixed by modifying all lines of the form
recon[i] = x + y;
into
recon[i] = (x + y) & 0xFF;

There are 8 lines which should be modified in the unFilterScanline function.

Here is a modified version of unFilterScanline:

void unFilterScanline(unsigned char* recon, const unsigned char* scanline, const unsigned char* precon, size_t bytewidth, unsigned long filterType, size_t length)
    {
      switch(filterType)
      {
        case 0: for(size_t i = 0; i < length; i++) recon[i] = scanline[i]; break;
        case 1:
          for(size_t i =         0; i < bytewidth; i++) recon[i] = scanline[i];
          for(size_t i = bytewidth; i <    length; i++) recon[i] = (scanline[i] + recon[i - bytewidth]) & 0xFF;
          break;
        case 2:
          if(precon) for(size_t i = 0; i < length; i++) recon[i] = (scanline[i] + precon[i]) & 0xFF;
          else       for(size_t i = 0; i < length; i++) recon[i] = scanline[i];
          break;
        case 3:
          if(precon)
          {
            for(size_t i =         0; i < bytewidth; i++) recon[i] = (scanline[i] + precon[i] / 2) & 0xFF;
            for(size_t i = bytewidth; i <    length; i++) recon[i] = (scanline[i] + ((recon[i - bytewidth] + precon[i]) / 2)) & 0xFF;
          }
          else
          {
            for(size_t i =         0; i < bytewidth; i++) recon[i] = scanline[i];
            for(size_t i = bytewidth; i <    length; i++) recon[i] = (scanline[i] + recon[i - bytewidth] / 2) & 0xFF;
          }
          break;
        case 4:
          if(precon)
          {
            for(size_t i =         0; i < bytewidth; i++) recon[i] = (scanline[i] + paethPredictor(0, precon[i], 0)) & 0xFF;
            for(size_t i = bytewidth; i <    length; i++) recon[i] = (scanline[i] + paethPredictor(recon[i - bytewidth], precon[i], precon[i - bytewidth])) & 0xFF;
          }
          else
          {
            for(size_t i =         0; i < bytewidth; i++) recon[i] = scanline[i];
            for(size_t i = bytewidth; i <    length; i++) recon[i] = (scanline[i] + paethPredictor(recon[i - bytewidth], 0, 0)) & 0xFF;
          }
          break;
        default: error = 36; return; //error: unexisting filter type given
      }
    }

[alur]

I fixed this in my fork.