ncpile: add roots element, track from bprev

src/lib/internal.h

@@ -303,6 +303,7 @@ typedef struct ncdirect {
 typedef struct ncpile {
   ncplane* top;               // topmost plane, never NULL
   ncplane* bottom;            // bottommost plane, never NULL
+  ncplane* roots;             // head of root plane list
   struct notcurses* nc;       // notcurses context
   struct ncpile *prev, *next; // circular list
   int dimy, dimx;             // rows and cols at time of render

src/lib/notcurses.c

@@ -304,7 +304,9 @@ make_ncpile(notcurses* nc, ncplane* n){
     ret->nc = nc;
     ret->top = n;
     ret->bottom = n;
-    if(nc->stdplane){
+    ret->roots = n;
+    n->bprev = &ret->roots;
+    if(nc->stdplane){ // stdplane (and thus stdpile) has already been created
       ret->prev = ncplane_pile(nc->stdplane)->prev;
       ncplane_pile(nc->stdplane)->prev->next = ret;
       ret->next = ncplane_pile(nc->stdplane);