Add configuration option for ghost reset distance. Improve documentation.

ghostshader.glsl

@@ -1,4 +1,5 @@
 #version 460 core
+// Billboarded ghost vertex shader: passes world pos/normal and flips UV.y for texture atlases.
 layout (location = 0) in vec3 aPos;
 layout (location = 1) in vec3 aNormal;
 layout (location = 2) in vec2 aTexCoord;