patch 8.1.2279: computation of highlight attributes is too complicated

Problem:    Computation of highlight attributes is too complicated.
Solution:   Simplify the attribute computation and make it more consistent.
            (closes #5190)  Fix that 'combine' set to zero doesn't work.
diff --git a/src/testdir/dumps/Test_textprop_01.dump b/src/testdir/dumps/Test_textprop_01.dump
index 24ca480..aabe25b 100644
--- a/src/testdir/dumps/Test_textprop_01.dump
+++ b/src/testdir/dumps/Test_textprop_01.dump
@@ -1,7 +1,7 @@
 | +0#af5f00255#ffffff0@1|1| |O+0#0000000&|n|e| +0&#ffff4012|t|w|o| +0&#ffffff0@63
 | +0#af5f00255&@1|2| |N+0#0000000#ffff4012|u|m|b|é|r| |1+0#4040ff13&|2|3| +0#0000000&|ä|n|d| |t|h|œ|n| |4+0#4040ff13&|¾|7|.+0#0000000&| +0&#ffffff0@46
 | +8#af5f00255&@1|3| >-+8#0000000#ffff4012|x+8&#ffffff0|a+8#4040ff13&@1|x+8#0000000&|-@1|x+8#4040ff13&|b@1|x+8#0000000&|-@1|x|c+8#4040ff13&@1|x|-+8#0000000&@1|x+8#4040ff13&|d@1|x|-+8#0000000&@1| @45
-| +0#af5f00255&@1|4| |/+0#40ff4011&@1| |c|o|m@1|e|n|t| |w+0&#e0e0e08|i|t|h| |e+8&&|r@1|o|r| +0&#ffffff0|i|n| |i|t| +0#0000000&@43
+| +0#af5f00255&@1|4| |/+0#40ff4011&@1| |c|o|m+0#0000000#e0e0e08@1|e|n+0#40ff4011#ffffff0|t| |w+0&#e0e0e08|i|t|h| |e+8&&|r@1|o|r| +0&#ffffff0|i|n| |i|t| +0#0000000&@43
 | +0#af5f00255&@1|5| |f+0#0000000&|i|r|s|t| |l+0&#ffff4012|i|n|e| @1|s|e|c|o|n|d| +0&#ffffff0|l|i|n|e| @1|t|h|i|r|d| |l|i|n|e| |f|o|u|r|t|h| |l+0&#ffff4012|i|n|e| +0&#ffffff0@23
 |~+0#4040ff13&| @73
 |~| @73
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index b09e1a7..002933d 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -678,8 +678,9 @@
 	\ "call prop_type_add('start', {'highlight': 'NumberProp', 'start_incl': 1})",
 	\ "call prop_type_add('end', {'highlight': 'NumberProp', 'end_incl': 1})",
 	\ "call prop_type_add('both', {'highlight': 'NumberProp', 'start_incl': 1, 'end_incl': 1})",
-	\ "call prop_type_add('background', {'highlight': 'NumberProp', 'combine': 1})",
-	\ "eval 'background'->prop_type_change({'highlight': 'BackgroundProp'})",
+	\ "call prop_type_add('background', {'highlight': 'BackgroundProp', 'combine': 0})",
+	\ "call prop_type_add('backgroundcomb', {'highlight': 'NumberProp', 'combine': 1})",
+	\ "eval 'backgroundcomb'->prop_type_change({'highlight': 'BackgroundProp'})",
 	\ "call prop_type_add('error', {'highlight': 'UnderlineProp', 'combine': 1})",
 	\ "call prop_add(1, 4, {'end_lnum': 3, 'end_col': 3, 'type': 'long'})",
 	\ "call prop_add(2, 9, {'length': 3, 'type': 'number'})",
@@ -688,7 +689,8 @@
 	\ "call prop_add(3, 7, {'length': 2, 'type': 'start'})",
 	\ "call prop_add(3, 11, {'length': 2, 'type': 'end'})",
 	\ "call prop_add(3, 15, {'length': 2, 'type': 'both'})",
-	\ "call prop_add(4, 12, {'length': 10, 'type': 'background'})",
+	\ "call prop_add(4, 6, {'length': 3, 'type': 'background'})",
+	\ "call prop_add(4, 12, {'length': 10, 'type': 'backgroundcomb'})",
 	\ "call prop_add(4, 17, {'length': 5, 'type': 'error'})",
 	\ "call prop_add(5, 7, {'length': 4, 'type': 'long'})",
 	\ "call prop_add(6, 1, {'length': 8, 'type': 'long'})",