ncplane_erase: fix bug in basecell preservation

USAGE.md

@@ -544,7 +544,7 @@ int ncplane_mergedown(struct ncplane* restrict src, struct ncplane* restrict dst
 // Erase every cell in the ncplane, resetting all attributes to normal, all
 // colors to the default color, and all cells to undrawn. All cells associated
 // with this ncplane are invalidated, and must not be used after the call,
-// excluding the base cell.
+// excluding the base cell. The cursor is homed.
 void ncplane_erase(struct ncplane* n);
 ```
 

doc/man/man3/notcurses_plane.3.md

@@ -196,6 +196,9 @@ expressed relative to the standard plane, and returns coordinates relative to
 of the rendering region. Only those cells where **src** intersects with **dst**
 might see changes. It is an error to merge a plane onto itself.
 
+**ncplane_erase** zeroes out every cell of the plane, dumps the egcpool, and
+homes the cursor. The base cell is preserved.
+
 ## Scrolling
 
 All planes, including the standard plane, are created with scrolling disabled.

include/notcurses/notcurses.h

@@ -1629,7 +1629,7 @@ API int ncplane_mergedown(struct ncplane* RESTRICT src, struct ncplane* RESTRICT
 // Erase every cell in the ncplane, resetting all attributes to normal, all
 // colors to the default color, and all cells to undrawn. All cells associated
 // with this ncplane is invalidated, and must not be used after the call,
-// excluding the base cell.
+// *excluding* the base cell. The cursor is homed.
 API void ncplane_erase(struct ncplane* n);
 
 #define NCPALETTESIZE 256

src/lib/notcurses.c

@@ -1773,8 +1773,10 @@ void ncplane_erase(ncplane* n){
   memset(n->fb, 0, sizeof(*n->fb) * n->lenx * n->leny);
   egcpool_dump(&n->pool);
   egcpool_init(&n->pool);
+  cell_init(&n->basecell);
   cell_load(n, &n->basecell, egc);
   free(egc);
+  n->y = n->x = 0;
 }
 
 void notcurses_cursor_enable(notcurses* nc){