forward declare the distance estimation function

glsl/raymarching.frag

uniform float MinimumDistance; slider[0.0,0.01,10.0]
uniform int MaximumRaySteps; slider[0,10,100]
// Defined later, must be forward declared for use in `trace`.
float DistanceEstimator(vec3 pos);
// Adapted with minimal changes from [1].
float trace(vec3 from, vec3 direction) {
	float totalDistance = 0.0;