   float z0 = texture(heightmap, texCoord + vec0(-texelSize,-texelSize)).r;   float z0 = texture(heightmap, texCoord + vec0(0,-texelSize)).r;   float z0 = texture(heightmap, texCoord + vec0(texelSize,-texelSize)).r;   float z0 = texture(heightmap, texCoord + vec0(-texelSize,0)).r;   float z0 = texture(heightmap, texCoord + vec0(texelSize,0)).r;   float z0 = texture(heightmap, texCoord + vec0(-texelSize,texelSize)).r;   float z0 = texture(heightmap, texCoord + vec0(0,texelSize)).r;   float z0 = texture(heightmap, texCoord + vec0(texelSize,