Misc: Validate wallpaper pixmap & Documentation update

- Split Pixmap validation out to validate_pixmap(). Validate wallpaper
  Pixmap as well.

- Update README.md and man page.

compton.c

@@ -669,30 +669,6 @@ win_rounded_corners(session_t *ps, win *w) {
   cxfree(rects);
 }
 
-/**
- * Validate pixmap of a window, and destroy pixmap and picture if invalid.
- */
-static void
-win_validate_pixmap(session_t *ps, win *w) {
-  if (!w->paint.pixmap)
-    return;
-
-  // Detect whether the pixmap is valid with XGetGeometry. Well, maybe there
-  // are better ways.
-  bool invalid = false;
-  {
-    Window rroot = None;
-    int rx = 0, ry = 0;
-    unsigned rwid = 0, rhei = 0, rborder = 0, rdepth = 0;
-    invalid = (!XGetGeometry(ps->dpy, w->paint.pixmap, &rroot, &rx, &ry,
-          &rwid, &rhei, &rborder, &rdepth) || !rwid || !rhei);
-  }
-
-  // Destroy pixmap and picture, if invalid
-  if (invalid)
-    free_paint(ps, &w->paint);
-}
-
 /**
  * Add a pattern to a condition linked list.
  */
@@ -840,6 +816,10 @@ get_root_tile(session_t *ps) {
     free_winprop(&prop);
   }
 
+  // Make sure the pixmap we got is valid
+  if (pixmap && !validate_pixmap(ps, pixmap))
+    pixmap = None;
+
   // Create a pixmap if there isn't any
   if (!pixmap) {
     pixmap = XCreatePixmap(ps->dpy, ps->root, 1, 1, ps->depth);

compton.h

@@ -443,8 +443,32 @@ win_is_fullscreen(session_t *ps, const win *w) {
 static void
 win_rounded_corners(session_t *ps, win *w);
 
-static void
-win_validate_pixmap(session_t *ps, win *w);
+/**
+ * Validate a pixmap.
+ *
+ * Detect whether the pixmap is valid with XGetGeometry. Well, maybe there
+ * are better ways.
+ */
+static inline bool
+validate_pixmap(session_t *ps, Pixmap pxmap) {
+  if (!pxmap) return false;
+
+  Window rroot = None;
+  int rx = 0, ry = 0;
+  unsigned rwid = 0, rhei = 0, rborder = 0, rdepth = 0;
+  return XGetGeometry(ps->dpy, pxmap, &rroot, &rx, &ry,
+        &rwid, &rhei, &rborder, &rdepth) && rwid && rhei;
+}
+
+/**
+ * Validate pixmap of a window, and destroy pixmap and picture if invalid.
+ */
+static inline void
+win_validate_pixmap(session_t *ps, win *w) {
+  // Destroy pixmap and picture, if invalid
+  if (!validate_pixmap(ps, w->paint.pixmap))
+    free_paint(ps, &w->paint);
+}
 
 /**
  * Wrapper of c2_match().