introduce drw_fontset_getwidth_clamp()

getting the width of a string is an O(n) operation, and in many cases
users only care about getting the width upto a certain number.

instead of calling drw_fontset_getwidth() and *then* clamping the
result, this patch introduces drw_fontset_getwidth_clamp() function,
similar to strnlen(), which will stop once we reach n.

the `invert` parameter was overloaded internally to preserve the API,
however library users should be calling drw_fontset_getwidth_clamp() and
not depend upon internal behavior of drw_text().

drw.c

@@ -268,7 +268,7 @@ drw_text(Drw *drw, int x, int y, unsigned int w, unsigned int h, unsigned int lp
 		return 0;
 
 	if (!render) {
-		w = ~w;
+		w = invert ? invert : ~invert;
 	} else {
 		XSetForeground(drw->dpy, drw->gc, drw->scheme[invert ? ColFg : ColBg].pixel);
 		XFillRectangle(drw->dpy, drw->drawable, drw->gc, x, y, w, h);
@@ -300,7 +300,13 @@ drw_text(Drw *drw, int x, int y, unsigned int w, unsigned int h, unsigned int lp
 
 					if (ew + tmpw > w) {
 						overflow = 1;
-						utf8strlen = ellipsis_len;
+						/* called from drw_fontset_getwidth_clamp():
+						 * it wants the width AFTER the overflow
+						 */
+						if (!render)
+							x += tmpw;
+						else
+							utf8strlen = ellipsis_len;
 					} else if (curfont == usedfont) {
 						utf8strlen += utf8charlen;
 						text += utf8charlen;
@@ -397,6 +403,15 @@ drw_fontset_getwidth(Drw *drw, const char *text)
 	return drw_text(drw, 0, 0, 0, 0, 0, text, 0);
 }
 
+unsigned int
+drw_fontset_getwidth_clamp(Drw *drw, const char *text, unsigned int n)
+{
+	unsigned int tmp = 0;
+	if (drw && drw->fonts && text && n)
+		tmp = drw_text(drw, 0, 0, 0, 0, 0, text, n);
+	return MIN(n, tmp);
+}
+
 void
 drw_font_getexts(Fnt *font, const char *text, unsigned int len, unsigned int *w, unsigned int *h)
 {

drw.h

@@ -35,6 +35,7 @@ void drw_free(Drw *drw);
 Fnt *drw_fontset_create(Drw* drw, const char *fonts[], size_t fontcount);
 void drw_fontset_free(Fnt* set);
 unsigned int drw_fontset_getwidth(Drw *drw, const char *text);
+unsigned int drw_fontset_getwidth_clamp(Drw *drw, const char *text, unsigned int n);
 void drw_font_getexts(Fnt *font, const char *text, unsigned int len, unsigned int *w, unsigned int *h);
 
 /* Colorscheme abstraction */