Fix tiling issue described in Airblader#22

Currently, containers only consider their neighbors and screen edges
If >2 containers are in a line, the outer containers adjust from outer gaps, but the middle containers know nothing of this and only consider the inner gaps
When the outer gaps differ substantially from the inner gaps, the outer containers are smaller as only they adjust for the larger outer gaps

With this change, the containers stop considering the edges, and instead the workspace as a whole is inset before any containers
The result is that many tiled containers have the same size, and the gaps overall work as the user might expect them to

src/render.c

@@ -50,16 +50,28 @@ void render_con(Con *con, bool render_fullscreen, bool already_inset) {
          (render_fullscreen ? "fullscreen " : ""), con, con->name, con->layout,
          params.children);
 
+    if (con->type == CT_WORKSPACE) {
+        gaps_t gaps = calculate_effective_gaps(con);
+        Rect inset = {
+                gaps.left,
+                gaps.top,
+                -(gaps.left + gaps.right),
+                -(gaps.top + gaps.bottom)
+        };
+        con->rect = rect_add(con->rect, inset);
+        params.rect = rect_add(params.rect, inset);
+        params.x += gaps.left;
+        params.y += gaps.top;
+    }
+
     bool should_inset = should_inset_con(con, params.children);
     if (!already_inset && should_inset) {
         gaps_t gaps = calculate_effective_gaps(con);
         Rect inset = (Rect){
-            has_adjacent_container(con, D_LEFT) ? gaps.inner : gaps.left,
-            has_adjacent_container(con, D_UP) ? gaps.inner : gaps.top,
-            has_adjacent_container(con, D_RIGHT) ? -gaps.inner : -gaps.right,
-            has_adjacent_container(con, D_DOWN) ? -gaps.inner : -gaps.bottom};
-        inset.width -= inset.x;
-        inset.height -= inset.y;
+            has_adjacent_container(con, D_LEFT) ? gaps.inner : 0,
+            has_adjacent_container(con, D_UP) ? gaps.inner : 0,
+            has_adjacent_container(con, D_RIGHT) ? -gaps.inner : 0,
+            has_adjacent_container(con, D_DOWN) ? -gaps.inner : 0};
 
         if (!render_fullscreen) {
             params.rect = rect_add(params.rect, inset);