Move win-agnostic parts of draw/drawregion to st.c

Introduces three functions to encapsulate X-specific behavior:
 * xdrawline: draws a portion of a single line (used by drawregion)
 * xbegindraw: called to prepare for drawing (will be useful for e.g.
   Wayland) and returns true if drawing should happen
 * xfinishdraw: called to finish drawing (used by draw)

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

st.c

@@ -166,6 +166,8 @@ static int32_t tdefcolor(int *, int *, int);
 static void tdeftran(char);
 static void tstrsequence(uchar);
 
+static void drawregion(int, int, int, int);
+
 static void selscroll(int, int);
 static void selsnap(int *, int *, int);
 
@@ -2526,6 +2528,29 @@ resettitle(void)
 	xsettitle(NULL);
 }
 
+void
+drawregion(int x1, int y1, int x2, int y2)
+{
+	int y;
+	for (y = y1; y < y2; y++) {
+		if (!term.dirty[y])
+			continue;
+
+		term.dirty[y] = 0;
+		xdrawline(term.line[y], x1, y, x2);
+	}
+}
+
+void
+draw(void)
+{
+	if (!xstartdraw())
+		return;
+	drawregion(0, 0, term.col, term.row);
+	xdrawcursor();
+	xfinishdraw();
+}
+
 void
 redraw(void)
 {

st.h

@@ -131,6 +131,7 @@ typedef union {
 
 void die(const char *, ...);
 void redraw(void);
+void draw(void);
 
 void iso14755(const Arg *);
 void printscreen(const Arg *);

win.h

@@ -23,12 +23,12 @@ enum win_mode {
 	                  |MODE_MOUSEMANY,
 };
 
-void draw(void);
-void drawregion(int, int, int, int);
-
 void xbell(void);
 void xclipcopy(void);
+void xdrawcursor(void);
+void xdrawline(Line, int, int, int);
 void xhints(void);
+void xfinishdraw(void);
 void xloadcols(void);
 int xsetcolorname(int, const char *);
 void xsettitle(char *);
@@ -36,3 +36,4 @@ int xsetcursor(int);
 void xsetmode(int, unsigned int);
 void xsetpointermotion(int);
 void xsetsel(char *);
+int xstartdraw(void);

x.c

@@ -129,7 +129,6 @@ static int xmakeglyphfontspecs(XftGlyphFontSpec *, const Glyph *, int, int, int)
 static void xdrawglyphfontspecs(const XftGlyphFontSpec *, Glyph, int, int, int);
 static void xdrawglyph(Glyph, int, int);
 static void xclear(int, int, int, int);
-static void xdrawcursor(void);
 static int xgeommasktogravity(int);
 static void xinit(void);
 static void cresize(int, int);
@@ -1512,59 +1511,51 @@ xsettitle(char *p)
 	XFree(prop.value);
 }
 
-void
-draw(void)
+int
+xstartdraw(void)
 {
-	drawregion(0, 0, term.col, term.row);
-	XCopyArea(xw.dpy, xw.buf, xw.win, dc.gc, 0, 0, win.w,
-			win.h, 0, 0);
-	XSetForeground(xw.dpy, dc.gc,
-			dc.col[IS_SET(MODE_REVERSE)?
-				defaultfg : defaultbg].pixel);
+	return IS_SET(MODE_VISIBLE);
 }
 
 void
-drawregion(int x1, int y1, int x2, int y2)
+xdrawline(Line line, int x1, int y1, int x2)
 {
-	int i, x, y, ox, numspecs;
+	int i, x, ox, numspecs;
 	Glyph base, new;
-	XftGlyphFontSpec *specs;
+	XftGlyphFontSpec *specs = xw.specbuf;
 
-	if (!(IS_SET(MODE_VISIBLE)))
-		return;
-
-	for (y = y1; y < y2; y++) {
-		if (!term.dirty[y])
+	numspecs = xmakeglyphfontspecs(specs, &line[x1], x2 - x1, x1, y1);
+	i = ox = 0;
+	for (x = x1; x < x2 && i < numspecs; x++) {
+		new = line[x];
+		if (new.mode == ATTR_WDUMMY)
 			continue;
-
-		term.dirty[y] = 0;
-
-		specs = xw.specbuf;
-		numspecs = xmakeglyphfontspecs(specs, &term.line[y][x1], x2 - x1, x1, y);
-
-		i = ox = 0;
-		for (x = x1; x < x2 && i < numspecs; x++) {
-			new = term.line[y][x];
-			if (new.mode == ATTR_WDUMMY)
-				continue;
-			if (selected(x, y))
-				new.mode ^= ATTR_REVERSE;
-			if (i > 0 && ATTRCMP(base, new)) {
-				xdrawglyphfontspecs(specs, base, i, ox, y);
-				specs += i;
-				numspecs -= i;
-				i = 0;
-			}
-			if (i == 0) {
-				ox = x;
-				base = new;
-			}
-			i++;
+		if (selected(x, y1))
+			new.mode ^= ATTR_REVERSE;
+		if (i > 0 && ATTRCMP(base, new)) {
+			xdrawglyphfontspecs(specs, base, i, ox, y1);
+			specs += i;
+			numspecs -= i;
+			i = 0;
+		}
+		if (i == 0) {
+			ox = x;
+			base = new;
 		}
-		if (i > 0)
-			xdrawglyphfontspecs(specs, base, i, ox, y);
+		i++;
 	}
-	xdrawcursor();
+	if (i > 0)
+		xdrawglyphfontspecs(specs, base, i, ox, y1);
+}
+
+void
+xfinishdraw(void)
+{
+	XCopyArea(xw.dpy, xw.buf, xw.win, dc.gc, 0, 0, win.w,
+			win.h, 0, 0);
+	XSetForeground(xw.dpy, dc.gc,
+			dc.col[IS_SET(MODE_REVERSE)?
+				defaultfg : defaultbg].pixel);
 }
 
 void