patch 8.2.1228: scrollbars not flush against the window edges when maximised

Problem:    Scrollbars not flush against the window edges when maximised.
Solution:   Add padding. (Ken Takata, closes #5602, closes #6466)
diff --git a/src/proto/gui_photon.pro b/src/proto/gui_photon.pro
index 9fcc512..5fcc9ba 100644
--- a/src/proto/gui_photon.pro
+++ b/src/proto/gui_photon.pro
@@ -18,6 +18,8 @@
 void gui_mch_settitle(char_u *title, char_u *icon);
 void gui_mch_set_scrollbar_thumb(scrollbar_T *sb, int val, int size, int max);
 void gui_mch_set_scrollbar_pos(scrollbar_T *sb, int x, int y, int w, int h);
+int gui_mch_get_scrollbar_xpadding(void);
+int gui_mch_get_scrollbar_ypadding(void);
 void gui_mch_create_scrollbar(scrollbar_T *sb, int orient);
 void gui_mch_enable_scrollbar(scrollbar_T *sb, int flag);
 void gui_mch_destroy_scrollbar(scrollbar_T *sb);