Misc: Add margin_t & Store frame extents with it & Misc

- Add margin_t, a structure that represents margins around a rectangle.

 - Store frame extents of a window with margin_t, replacing the 4
   fields ({left,right,top,bottom}_width) in struct _win.

 - Add two helper macros, cmemzero() and cmemzero_one(), that zero out a
   block of memory.

src/common.h

@@ -279,6 +279,17 @@ typedef struct {
   int y;
 } geometry_t;
 
+/// A structure representing margins around a rectangle.
+typedef struct {
+  int top;
+  int left;
+  int bottom;
+  int right;
+} margin_t;
+
+// Or use cmemzero().
+#define MARGIN_INIT { 0, 0, 0, 0 }
+
 /// Enumeration type of window painting mode.
 typedef enum {
   WMODE_TRANS,
@@ -1174,8 +1185,8 @@ typedef struct _win {
   // Frame-opacity-related members
   /// Current window frame opacity. Affected by window opacity.
   double frame_opacity;
-  /// Frame widths. Determined by client window attributes.
-  unsigned int left_width, right_width, top_width, bottom_width;
+  /// Frame extents. Acquired from _NET_FRAME_EXTENTS.
+  margin_t frame_extents;
 
   // Shadow-related members
   /// Whether a window has shadow. Calculated.
@@ -1343,6 +1354,13 @@ allocchk_(const char *func_name, void *ptr) {
 /// @brief Wrapper of ealloc().
 #define crealloc(ptr, nmemb, type) ((type *) allocchk(realloc((ptr), (nmemb) * sizeof(type))))
 
+/// @brief Zero out the given memory block.
+#define cmemzero(ptr, size) memset((ptr), 0, (size))
+
+/// @brief Wrapper of cmemzero() that handles a pointer to a single item, for
+///        convenience.
+#define cmemzero_one(ptr) cmemzero((ptr), sizeof(*(ptr)))
+
 /**
  * Return whether a struct timeval value is empty.
  */