Allow specifying alternate config files

 - The program now takes a single, optional, command line argument: the
   path to the config file to read.  If unspecified, it will continue to
   use the old config.txt.

 - Added dragon_curve.txt, a simpler example config.

README.md

@@ -48,17 +48,20 @@ keypresses:
 Configuring the L-System
 ========================
 
-The system reads its configuration from the "config.txt" file.  The file
-must contain only ASCII characters. It starts with the L-system definition
-(replacement rules). Following a line containing the keyword `actions`, the
-file switches to defining the drawing actions the "turtle" takes when
-encountering each character. (The `actions` line must occur exactly once in the
-config.)
+The system reads its configuration from the "config.txt" file by default. If
+desired, you can pass a path to a different config as the program's first (and
+only) command line argument.
+
+The config file must contain only ASCII characters. It starts with the L-system
+definition (replacement rules). Following a line containing the keyword
+`actions`, the file switches to defining the drawing actions the "turtle" takes
+when encountering each character. (The `actions` line must occur exactly once
+in the config.)
 
 If you want a longer example, the default "config.txt" contains instructions
 for drawing a multi-colored Sierpinski pyramid. A simpler, single, dragon-curve
-example can be obtained by replacing the contents of "config.txt" with the
-following lines:
+example is also available in `dragon_curve.txt`. The contents of
+`dragon_curve.txt` are copied here:
 ```
 init F
 F F+G

dragon_curve.txt

@@ -0,0 +1,15 @@
+# Copied from https://en.wikipedia.org/wiki/L-system#Example_6:_Dragon_curve
+init F
+F F+G
+G F-G
+
+actions
+F
+move_forward 1.0
+G
+move_forward 1.0
+-
+rotate -90.0
++
+rotate 90.0
+

l_system_3d.c

@@ -34,6 +34,7 @@ static void FreeApplicationState(ApplicationState *s) {
   if (s->mesh) DestroyLSystemMesh(s->mesh);
   if (s->turtle) DestroyTurtle3D(s->turtle);
   if (s->config) DestroyLSystemConfig(s->config);
+  free(s->config_file_path);
   glDeleteBuffers(1, &(s->ubo));
   if (s->window) glfwDestroyWindow(s->window);
   memset(s, 0, sizeof(*s));
@@ -203,20 +204,20 @@ static int DecreaseIterations(ApplicationState *s) {
   return 1;
 }
 
-// Reloads the config file from ./config.txt. If this fails, then we'll just
-// print a message and return. (The config can be faulty at runtime, but we
-// won't start the program unless it's OK.)
+// Reloads the config file. If this fails, then we'll just print a message and
+// return. (The config can be faulty at runtime, but we won't start the program
+// unless it's OK when starting.)
 static void ReloadConfig(ApplicationState *s) {
   LSystemConfig *new_config = NULL;
   LSystemConfig *old_config = s->config;
-  new_config = LoadLSystemConfig("./config.txt");
+  new_config = LoadLSystemConfig(s->config_file_path);
   if (!new_config) {
-    printf("Failed loading new config file.\n");
+    printf("Failed reloading the config file.\n");
     return;
   }
   s->config = new_config;
   DestroyLSystemConfig(old_config);
-  printf("Config updated OK.\n");
+  printf("Config %s updated OK.\n", s->config_file_path);
 }
 
 static void PrintMemoryUsage(ApplicationState *s) {
@@ -341,6 +342,22 @@ int main(int argc, char **argv) {
     printf("Failed allocating application state.\n");
     return 1;
   }
+  if (argc > 2) {
+    printf("Usage: %s [config file path]\n", argv[0]);
+    FreeApplicationState(s);
+    return 1;
+  }
+  if (argc == 2) {
+    s->config_file_path = strdup(argv[1]);
+  } else {
+    // Using strdup so that we can "free" it no matter what.
+    s->config_file_path = strdup("./config.txt");
+  }
+  if (!s->config_file_path) {
+    printf("Failed copying config file path.\n");
+    FreeApplicationState(s);
+    return 1;
+  }
   if (!SetupWindow(s)) {
     printf("Failed setting up window.\n");
     FreeApplicationState(s);
@@ -378,13 +395,13 @@ int main(int argc, char **argv) {
     to_return = 1;
     goto cleanup;
   }
-  s->config = LoadLSystemConfig("./config.txt");
+  s->config = LoadLSystemConfig(s->config_file_path);
   if (!s->config) {
-    printf("Error parsing ./config.txt.\n");
+    printf("Error parsing %s\n", s->config_file_path);
     to_return = 1;
     goto cleanup;
   }
-  printf("Config loaded OK!\n");
+  printf("Config %s loaded OK!\n", s->config_file_path);
   s->l_system_string = (uint8_t *) strdup(s->config->init);
   if (!s->l_system_string) {
     printf("Error initializing L-system string.\n");

l_system_3d.h

@@ -22,6 +22,7 @@ typedef struct {
   float aspect_ratio;
   double frame_start;
   double frame_duration;
+  char *config_file_path;
   LSystemMesh *mesh;
   LSystemConfig *config;
   Turtle3D *turtle;