optimization: Render on pixmaps and copy the result on Stack_Wins

This should speed up the rendering of Stack_Wins with many window
decorations and it should considerably reduce flicker.

src/xcb.c

@@ -18,6 +18,7 @@
 #include <xcb/xcb.h>
 #include <xcb/xcb_keysyms.h>
 
+#include "i3.h"
 #include "util.h"
 #include "xcb.h"
 
@@ -259,3 +260,35 @@ void xcb_raise_window(xcb_connection_t *conn, xcb_window_t window) {
         uint32_t values[] = { XCB_STACK_MODE_ABOVE };
         xcb_configure_window(conn, window, XCB_CONFIG_WINDOW_STACK_MODE, values);
 }
+
+/*
+ *
+ * Prepares the given Cached_Pixmap for usage (checks whether the size of the
+ * object this pixmap is related to (e.g. a window) has changed and re-creates
+ * the pixmap if so).
+ *
+ */
+void cached_pixmap_prepare(xcb_connection_t *conn, struct Cached_Pixmap *pixmap) {
+        LOG("preparing pixmap\n");
+
+        /* If the Rect did not change, the pixmap does not need to be recreated */
+        if (memcmp(&(pixmap->rect), pixmap->referred_rect, sizeof(Rect)) == 0)
+                return;
+
+        memcpy(&(pixmap->rect), pixmap->referred_rect, sizeof(Rect));
+
+        if (pixmap->id == 0 || pixmap->gc == 0) {
+                LOG("Creating new pixmap...\n");
+                pixmap->id = xcb_generate_id(conn);
+                pixmap->gc = xcb_generate_id(conn);
+        } else {
+                LOG("Re-creating this pixmap...\n");
+                xcb_free_gc(conn, pixmap->gc);
+                xcb_free_pixmap(conn, pixmap->id);
+        }
+
+        xcb_create_pixmap(conn, root_depth, pixmap->id,
+                          pixmap->referred_drawable, pixmap->rect.width, pixmap->rect.height);
+
+        xcb_create_gc(conn, pixmap->gc, pixmap->id, 0, 0);
+}