Improve inheritance of fan speed at height

It compensates for the '- layer_height_0' by adding +1 back after the division. Also use floor, since I find that more intuitive since the break points come up at exactly the slice heights instead of at 0.25, 0.35, 0.45, etc.

Contributes to issue CURA-2314.

resources/definitions/fdmprinter.def.json

@@ -2231,7 +2231,7 @@
                             "default_value": 1,
                             "minimum_value": "0",
                             "maximum_value_warning": "100",
-                            "value": "max(0, int(round((cool_fan_full_at_height - layer_height_0) / layer_height, 0)))",
+                            "value": "max(0, int(math.floor((cool_fan_full_at_height - layer_height_0) / layer_height) + 1))",
                             "settable_per_mesh": false,
                             "settable_per_extruder": true
                         }