patch 8.1.1323: 'mouse' option is reset when using GPM mouse

Problem:    'mouse' option is reset when using GPM mouse.
Solution:   Add flag for GPM mouse.
diff --git a/src/term.c b/src/term.c
index 29d4025..fb002ec 100644
--- a/src/term.c
+++ b/src/term.c
@@ -2108,8 +2108,9 @@
 #  define HMT_JSBTERM	8
 #  define HMT_PTERM	16
 #  define HMT_URXVT	32
-#  define HMT_SGR	64
-#  define HMT_SGR_REL	128
+#  define HMT_GPM	64
+#  define HMT_SGR	128
+#  define HMT_SGR_REL	256
 static int has_mouse_termcode = 0;
 # endif
 
@@ -2150,6 +2151,11 @@
 	has_mouse_termcode |= HMT_URXVT;
     else
 #   endif
+#   ifdef FEAT_MOUSE_GPM
+    if (n == KS_GPM_MOUSE)
+	has_mouse_termcode |= HMT_GPM;
+    else
+#   endif
     if (n == KS_SGR_MOUSE)
 	has_mouse_termcode |= HMT_SGR;
     else if (n == KS_SGR_MOUSE_RELEASE)
@@ -2197,6 +2203,11 @@
 	has_mouse_termcode &= ~HMT_URXVT;
     else
 #   endif
+#   ifdef FEAT_MOUSE_GPM
+    if (n == KS_GPM_MOUSE)
+	has_mouse_termcode &= ~HMT_GPM;
+    else
+#   endif
     if (n == KS_SGR_MOUSE)
 	has_mouse_termcode &= ~HMT_SGR;
     else if (n == KS_SGR_MOUSE_RELEASE)