add notcurses_stddim_yx_const()

NEWS.md

@@ -9,6 +9,7 @@ rearrangements of Notcurses.
   * Add `ncinput_equal_p()` for comparison of `ncinput` structure data.
   * `ncmenu_offer_input()` now recognizes the shortcuts for registered
     sections, and will unroll the appropriate section when given input.
+  * Added `notcurses_stddim_yx_const()` (`notcurses_stddim_yx()` `const` form).
 
 * 2.0.0 (2020-10-12) "Stankonia"
   * **API STABILITY!** The API expressed in 2.0.0 will be maintained throughout

USAGE.md

@@ -207,6 +207,14 @@ notcurses_stddim_yx(struct notcurses* nc, int* restrict y, int* restrict x){
   ncplane_dim_yx(s, y, x); // accepts NULL
   return s;
 }
+
+static inline const struct ncplane*
+notcurses_stddim_yx_const(const struct notcurses* nc, int* restrict y, int* restrict x){
+  const struct ncplane* s = notcurses_stdplane_const(nc); // can't fail
+  ncplane_dim_yx(s, y, x); // accepts NULL
+  return s;
+}
+
 ```
 
 A reference to the standard plane *is* persistent across a screen resize, as are

doc/man/man3/notcurses_stdplane.3.md

@@ -16,6 +16,8 @@ notcurses_stdplane - acquire the standard ncplane
 
 **static inline struct ncplane* notcurses_stddim_yx(struct notcurses* nc, int* restrict y, int* restrict x);**
 
+**static inline const struct ncplane* notcurses_stddim_yx_const(const struct notcurses* nc, int* restrict y, int* restrict x);**
+
 # DESCRIPTION
 
 **notcurses_stdplane** returns a handle to the standard ncplane for the context

include/notcurses/notcurses.h

@@ -991,6 +991,13 @@ notcurses_stddim_yx(struct notcurses* nc, int* RESTRICT y, int* RESTRICT x){
   return s;
 }
 
+static inline const struct ncplane*
+notcurses_stddim_yx_const(const struct notcurses* nc, int* RESTRICT y, int* RESTRICT x){
+  const struct ncplane* s = notcurses_stdplane_const(nc); // can't fail
+  ncplane_dim_yx(s, y, x); // accepts NULL
+  return s;
+}
+
 static inline int
 ncplane_dim_y(const struct ncplane* n){
   int dimy;