patch 8.1.1400: using global pointer for tab-local popups is clumsy

Problem:    Using global pointer for tab-local popups is clumsy.
Solution:   Use the pointer in tabpage_T.
diff --git a/src/screen.c b/src/screen.c
index 00894ce..30e1bb2 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -610,7 +610,7 @@
     }
 #ifdef FEAT_TEXT_PROP
     // TODO: avoid redrawing everything when there is a popup window.
-    if (first_popupwin != NULL || first_tab_popupwin != NULL)
+    if (first_popupwin != NULL || curtab->tp_first_popupwin != NULL)
 	type = NOT_VALID;
 #endif
 
@@ -1000,7 +1000,7 @@
     // Reset all the VALID_POPUP flags.
     for (wp = first_popupwin; wp != NULL; wp = wp->w_next)
 	wp->w_valid &= ~VALID_POPUP;
-    for (wp = first_tab_popupwin; wp != NULL; wp = wp->w_next)
+    for (wp = curtab->tp_first_popupwin; wp != NULL; wp = wp->w_next)
 	wp->w_valid &= ~VALID_POPUP;
 
     // TODO: don't redraw every popup every time.
@@ -1018,7 +1018,7 @@
 		lowest_zindex = wp->w_zindex;
 		lowest_wp = wp;
 	    }
-	for (wp = first_tab_popupwin; wp != NULL; wp = wp->w_next)
+	for (wp = curtab->tp_first_popupwin; wp != NULL; wp = wp->w_next)
 	    if ((wp->w_valid & VALID_POPUP) == 0
 					       && wp->w_zindex < lowest_zindex)
 	    {