Fix wrong value for VIPS_FOREIGN_PNG_FILTER_ALL
Fix wrong value for VIPS_FOREIGN_PNG_FILTER_ALL

The value is derived by oring all the filter flags:

    0x08 | 0x10 | 0x20 | 0x40 | 0x80 == 0xf8

The value was also checked against PNG_ALL_FILTERS in png.h.

lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips