X11: add necessary XFree call after call to XGetKeyboardMapping (#1427)

Also :
- remove duplication of function keycode_to_ascii for X11 and Wayland;
- add comments detailing implementation.

src/Fl_x.cxx

@@ -1301,17 +1301,6 @@ static bool remove_xid_vector(Window xid) {
 }
 
 
-static char keycode_to_ascii(int keycode) {
-  static const char row_AD[] = "qwertyuiop";
-  static const char row_AC[] = "asdfghjkl";
-  static const char row_AB[] = "zxcvbnm";
-  if (keycode >= 24 && keycode <= 33) return row_AD[keycode - 24];
-  else if (keycode >= 38 && keycode <= 46) return row_AC[keycode - 38];
-  else if (keycode >= 52 && keycode <= 58) return row_AB[keycode - 52];
-  else return 0;
-}
-
-
 int fl_handle(const XEvent& thisevent)
 {
   XEvent xevent = thisevent;
@@ -2006,13 +1995,15 @@ int fl_handle(const XEvent& thisevent)
     int keysyms_per_keycode;
     KeySym *syms = XGetKeyboardMapping(fl_display, 38 /* 'A' key on US keyboard */, 1,
                                         &keysyms_per_keycode);
-    // Check for non-Latin keyboard layout
+    // Check for non-Latin keyboard layout. Based on the assumption that the 'A' key of the keyboard
+    // having a non-ASCII keysym indicates a non-Latin layout.
     if (syms[0] > 'z') {
-      int asciiSym = keycode_to_ascii(keycode);
+      int asciiSym = Fl_Unix_System_Driver::keycode_to_ascii(keycode);
       if (asciiSym != 0) {
         keysym = asciiSym;
       }
     }
+    XFree(syms);
 
     // We have to get rid of the XK_KP_function keys, because they are
     // not produced on Windoze and thus case statements tend not to check

src/drivers/Unix/Fl_Unix_System_Driver.H

@@ -49,6 +49,7 @@ public:
   static unsigned char *create_bmp(const unsigned char *data, int W, int H, int *return_size);
   static Fl_RGB_Image *own_bmp_to_RGB(char *bmp);
   static void read_int(uchar *c, int& i);
+  static char keycode_to_ascii(int keycode);
 };
 
 #endif /* FL_NIX_SYSTEM_DRIVER_H */

src/drivers/Unix/Fl_Unix_System_Driver.cxx

@@ -912,3 +912,23 @@ Fl_RGB_Image *Fl_Unix_System_Driver::own_bmp_to_RGB(char *bmp) {
   img->alloc_array = 1;
   return img;
 }
+
+
+/*
+ Used with non-Latin keyboards to get the Latin letter associated to a key,
+ typically useful when used with Ctrl.
+ Based on the assumption that Latin letters have the QWERTY layout in any
+ non-Latin layout. This assumption was verified true for Greek, Russian,
+ and Thai layouts.
+ Not used with keyboard layouts using Latin letters where the location
+ of letters varies extensively (e.g., French, Turkish).
+ */
+char Fl_Unix_System_Driver::keycode_to_ascii(int keycode) {
+  static const char row_AD[] = "qwertyuiop";
+  static const char row_AC[] = "asdfghjkl";
+  static const char row_AB[] = "zxcvbnm";
+  if (keycode >= 24 && keycode <= 33) return row_AD[keycode - 24];
+  else if (keycode >= 38 && keycode <= 46) return row_AC[keycode - 38];
+  else if (keycode >= 52 && keycode <= 58) return row_AB[keycode - 52];
+  else return 0;
+}

src/drivers/Wayland/Fl_Wayland_Screen_Driver.cxx

@@ -21,7 +21,7 @@
 #include <wayland-cursor.h>
 #include "../../../libdecor/build/fl_libdecor.h"
 #include "xdg-shell-client-protocol.h"
-#include "../Posix/Fl_Posix_System_Driver.H"
+#include "../Unix/Fl_Unix_System_Driver.H"
 #include <FL/Fl.H>
 #include <FL/Fl_Image_Surface.H>
 #include <FL/platform.H>
@@ -589,17 +589,6 @@ static void remove_int_vector(std::vector<int>& v, int val) {
 }
 
 
-static char keycode_to_ascii(int keycode) {
-  static const char row_AD[] = "qwertyuiop";
-  static const char row_AC[] = "asdfghjkl";
-  static const char row_AB[] = "zxcvbnm";
-  if (keycode >= 24 && keycode <= 33) return row_AD[keycode - 24];
-  else if (keycode >= 38 && keycode <= 46) return row_AC[keycode - 38];
-  else if (keycode >= 52 && keycode <= 58) return row_AB[keycode - 52];
-  else return 0;
-}
-
-
 static int process_wld_key(struct xkb_state *xkb_state, uint32_t key,
                            uint32_t *p_keycode, xkb_keysym_t *p_sym) {
   uint32_t keycode = key + 8;
@@ -615,9 +604,10 @@ static int process_wld_key(struct xkb_state *xkb_state, uint32_t key,
   } else if (keycode == 19) {
     for_key_vector = '0';
   }
-  // Check for non-Latin keyboard layout
+  // Check for non-Latin keyboard layout. Based on the assumption that the 'A' key of the keyboard
+  // having a non-ASCII keysym indicates a non-Latin layout.
   if (xkb_state_key_get_one_sym(xkb_state, 38 /* 'A' key on US keyboard */) > 'z') {
-    int asciiSym = keycode_to_ascii(keycode);
+    int asciiSym = Fl_Unix_System_Driver::keycode_to_ascii(keycode);
     if (asciiSym != 0) {
       for_key_vector = asciiSym;
     }