Add redisplay-can-quit to keep redisplay atomic by default

* src/xdisp.c (redisplay_internal): Bind inhibit-quit when
redisplay-can-quit is nil to ensure atomic redisplay updates.
(syms_of_xdisp): New variable redisplay-can-quit.
* etc/NEWS: Document the new variable.

etc/NEWS

@@ -2455,6 +2455,13 @@ Binding 'inhibit-message' to a non-nil value will now suppress both
 the display of messages and the clearing of the echo area, such as
 caused by calling 'message' with a nil argument.
 
+---
+** New variable 'redisplay-can-quit' to allow quit during redisplay.
+This variable is normally nil to ensure redisplay completes atomically.
+Setting it to t allows C-g to interrupt redisplay, which can be useful
+for debugging redisplay hangs but may leave the display in an
+inconsistent state.
+
 ** Special Events
 
 +++

src/xdisp.c

@@ -17160,6 +17160,9 @@ redisplay_internal (void)
   /* Record a function that clears redisplaying_p
      when we leave this function.  */
   specpdl_ref count = SPECPDL_INDEX ();
+  if (!redisplay_can_quit)
+    specbind (Qinhibit_quit, Qt);
+
   record_unwind_protect_void (unwind_redisplay);
   redisplaying_p = true;
   block_buffer_flips ();
@@ -38753,6 +38756,14 @@ The recommended non-zero value is between 100000 and 1000000,
 depending on your patience and the speed of your system.  */);
   max_redisplay_ticks = 0;
 
+  DEFVAR_BOOL ("redisplay-can-quit",
+	       redisplay_can_quit,
+	       doc: /* If true allow quit to interrupt redisplay.
+Normally, we inhibit quit in redisplay to ensure each update completes
+atomically and leaves the display in a known-good state.  You may want to
+set this variable to t to help debug redisplay hangs.  */);
+  redisplay_can_quit = false;
+
   /* Called by decode_mode_spec.  */
   DEFSYM (Qfile_remote_p, "file-remote-p");