Feature: Configuration file parsing

- Add support for parsing configuration files using libconfig.
  (Dependency on libconfig could be made optional once we get some
  better building system.) Few tests has been done, bugs to be expected.
  compton searches for a configuration file mostly according to the XDG
  standard. Firstly the configuration file requested by --config, then
  $XDG_CONFIG_HOME/compton.conf (~/.config/compton.conf, usually), then
  ~/.compton.conf, then compton.conf under $XDG_DATA_DIRS (often
  /etc/xdg/compton.conf). A sample configuration file is supplied as
  compton.sample.conf. Configuration file syntax may change in the
  future.  Commandline switches has higher priority than configuration
  file, except for --shadow-exclude. Use --config /dev/null to
  temporarily disable configuration file.

- Fix a bug that causes windows to disappear or be partially rendered on
  opacity changes.

- Fix a bug that causes some windows to ignore -i (inactive_opacity) and
  --inactive-dim, caused by the default window type change in
  a5d9955ca4.

src/compton.h

@@ -24,6 +24,8 @@
 // Whether to enable JIT support of libpcre. This may cause problems on PaX
 // kernels.
 #define CONFIG_REGEX_PCRE_JIT 1
+// Whether to enable parsing of configuration files using libconfig
+#define CONFIG_LIBCONFIG 1
 
 // === Includes ===
 
@@ -44,10 +46,16 @@
 #include <locale.h>
 
 #include <fnmatch.h>
+
 #ifdef CONFIG_REGEX_PCRE
 #include <pcre.h>
 #endif
 
+#ifdef CONFIG_LIBCONFIG
+#include <libgen.h>
+#include <libconfig.h>
+#endif
+
 #include <X11/Xlib.h>
 #include <X11/Xutil.h>
 #include <X11/Xatom.h>
@@ -277,6 +285,34 @@ mstrcpy(const char *src) {
   return str;
 }
 
+/**
+ * Allocate the space and join two strings.
+ */
+static inline char *
+mstrjoin(const char *src1, const char *src2) {
+  char *str = malloc(sizeof(char) * (strlen(src1) + strlen(src2) + 1));
+
+  strcpy(str, src1);
+  strcat(str, src2);
+
+  return str;
+}
+
+/**
+ * Allocate the space and join two strings;
+ */
+static inline char *
+mstrjoin3(const char *src1, const char *src2, const char *src3) {
+  char *str = malloc(sizeof(char) * (strlen(src1) + strlen(src2)
+        + strlen(src3) + 1));
+
+  strcpy(str, src1);
+  strcat(str, src2);
+  strcat(str, src3);
+
+  return str;
+}
+
 /**
  * Normalize an int value to a specific range.
  *
@@ -285,13 +321,29 @@ mstrcpy(const char *src) {
  * @param max maximum value
  * @return normalized value
  */
-static inline double
+static inline int
 normalize_i_range(int i, int min, int max) {
   if (i > max) return max;
   if (i < min) return min;
   return i;
 }
 
+/**
+ * Select the larger integer of two.
+ */
+static inline int
+max_i(int a, int b) {
+  return (a > b ? a : b);
+}
+
+/**
+ * Select the smaller integer of two.
+ */
+static inline int
+min_i(int a, int b) {
+  return (a > b ? b : a);
+}
+
 /**
  * Normalize a double value to a specific range.
  *
@@ -530,6 +582,12 @@ static Picture
 solid_picture(Display *dpy, Bool argb, double a,
               double r, double g, double b);
 
+static inline bool is_normal_win(const win *w) {
+  return (WINTYPE_NORMAL == w->window_type
+      || WINTYPE_UTILITY == w->window_type
+      || WINTYPE_UNKNOWN == w->window_type);
+}
+
 static bool
 win_match_once(win *w, const wincond *cond);
 
@@ -789,5 +847,16 @@ ev_handle(XEvent *ev);
 static void
 fork_after(void);
 
+#ifdef CONFIG_LIBCONFIG
+static FILE *
+open_config_file(char *cpath, char **path);
+
+static void
+parse_config(char *cpath);
+#endif
+
+static void
+get_cfg(int argc, char *const *argv);
+
 static void
 get_atoms(void);