always render at least *something*

glsl/raymarching.frag

#group DistranceEstimator
uniform float MinimumDistance; slider[0.0,0.01,10.0]
uniform int MaximumRaySteps; slider[0,10,100]
uniform int MaximumRaySteps; slider[1,10,100]
// Defined later, must be forward declared for use in `trace`.
float DistanceEstimator(vec3 pos);