v0.1.8

Sign on a cactus, ice/snow melting, reference random tick

• isSolidMaterial now switches on tileMaterial() instead of the collision box, so cactus and cobweb take a sign again; fire got its own MAT_FIRE.
• ports IceTile::playerDestroy: ice broken over solid or liquid leaves water.
• ports the Ice/Snow/TopSnow melt ticks, on block light only.
• tileRandomTick picks cells with MCPE/LCE's own LCG (one step, x/z/y sliced 8 bits apart) instead of three rand() calls.