Visual: highlight char-at-cursor

Decide whether to highlight the visual-selected character under the
cursor, depending on 'guicursor' style:

- Highlight if cursor is blinking or non-block (vertical, horiz).
- Do NOT highlight if cursor is non-blinking block.

Traditionally Vim's visual selection does "reverse mode", which perhaps
conflicts with the non-blinking block cursor. But 'guicursor' defaults
to a vertical bar for selection=exclusive, and this confuses users who
expect to see the text highlighted.

closes #8983

src/nvim/cursor_shape.c

@@ -254,6 +254,16 @@ char_u *parse_shape_opt(int what)
   return NULL;
 }
 
+/// Returns true if the cursor is non-blinking "block" shape during
+/// visual selection.
+///
+/// @param exclusive If 'selection' option is "exclusive".
+bool cursor_is_block_during_visual(bool exclusive)
+{
+  int mode_idx = exclusive ? SHAPE_IDX_VE : SHAPE_IDX_V;
+  return (SHAPE_BLOCK == shape_table[mode_idx].shape
+          && 0 == shape_table[mode_idx].blinkon);
+}
 
 /// Map cursor mode from string to integer
 ///