Add configuration option for ghost reset distance. Improve documentation.

shader.cpp

@@ -5,6 +5,7 @@
 
 #include "shader.h"
 
+// Load a GLSL file from disk and compile it as vertex/fragment/etc.
 int create_shader(const char *filename, int shadertype) {
     FILE* fp = fopen(filename, "r");
     if (!fp) {
@@ -38,6 +39,7 @@ int create_shader(const char *filename, int shadertype) {
     return shader;
 }
 
+// Compile and link a shader program from the given vertex/fragment GLSL files.
 int create_shader_program(const char *vertex_filename, const char *fragment_filename) {
 
     int vertex_shader = create_shader(vertex_filename, GL_VERTEX_SHADER);
@@ -70,6 +72,7 @@ int create_shader_program(const char *vertex_filename, const char *fragment_file
 
 }
 
+// Simple wrappers to make uniform updates less verbose at call sites.
 void set_uniform(int program, const char *key, float value) {
     glUniform1f(glGetUniformLocation(program, key), value);
 }