Add configuration option for ghost reset distance. Improve documentation.

shader.h

@@ -3,6 +3,7 @@
 
 #include <glm/glm.hpp>
 
+// Utility helpers for compiling GLSL source files and setting uniforms.
 int create_shader(const char *filename, int shadertype);
 int create_shader_program(const char *vertex_filename, const char *fragment_filename);