demo: Fix strict aliasing issues with gcc-7.3
demo: Fix strict aliasing issues with gcc-7.3

hashRange reads unsigned int from the given byte range, which violates
strict aliasing as they refer to floats. Work around this by using the
union hack (which technically violates strict aliasing, but is blessed
by gcc).

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer