windowarea_margin patch for dwm 5.9

Introduce margin around the "window area", to eliminate "double gaps" between
client windows by doubling it on the window area edges too.
m->gap is the gap in pixel around window area and client windows.
The default value of this gap should be defined in config.h:

static const unsigned int gappx = 3;

Note that due to "double gaps", the gaps you see are the double size of gappx.

diff -up a/dwm.c b/dwm.c
--- a/dwm.c	2011-08-29 12:00:00.00 +0000
+++ b/dwm.c	2011-08-29 12:00:00.00 +0000
@@ -125,6 +125,7 @@ typedef struct {
 struct Monitor {
 	char ltsymbol[16];
 	float mfact;
+	int gap;
 	int num;
 	int by;               /* bar geometry */
 	int mx, my, mw, mh;   /* screen size */
@@ -1773,12 +1774,16 @@ updatebars(void) {

 void
 updatebarpos(Monitor *m) {
-	m->wy = m->my;
-	m->wh = m->mh;
+	m->wy = m->my + m->gap;
+	m->wh = m->mh - m->gap * 2;
 	if(m->showbar) {
 		m->wh -= bh;
-		m->by = m->topbar ? m->wy : m->wy + m->wh;
-		m->wy = m->topbar ? m->wy + bh : m->wy;
+		if(m->topbar) {
+			m->by = m->my;
+			m->wy += bh;
+		}
+		else
+			m->by = m->wy + m->wh + m->gap;
 	}
 	else
 		m->by = -bh;
@@ -1819,12 +1824,15 @@ updategeom(void) {
 				    || unique[i].width != m->mw || unique[i].height != m->mh))
 				{
 					dirty = True;
+					m->gap = gappx;
 					m->num = i;
 					m->mx = m->wx = unique[i].x_org;
 					m->my = m->wy = unique[i].y_org;
 					m->mw = m->ww = unique[i].width;
 					m->mh = m->wh = unique[i].height;
 					updatebarpos(m);
+					m->wx += m->gap;
+					m->ww -= m->gap * 2;
 				}
 		}
 		else { /* less monitors available nn < n */
@@ -1854,9 +1862,12 @@ updategeom(void) {
 			mons = createmon();
 		if(mons->mw != sw || mons->mh != sh) {
 			dirty = True;
+			mons->gap = gappx;
 			mons->mw = mons->ww = sw;
 			mons->mh = mons->wh = sh;
 			updatebarpos(mons);
+			mons->wx += mons->gap;
+			mons->ww -= mons->gap * 2;
 		}
 	}
 	if(dirty) {