Fix a rare platform-specific crash when operating on opacity.

[AbnormalPoof]

start and end would sometimes be integers, but still fall into the VariantType switch case. This would cause a memory access violation on macOS specifically!

This PR adds more strict type checking, which fixes the crash entirely.

[EliteMasterEric]

While operating on the properties of a target component, the Actuator class will enter this block:

            var isVariant = false;
            if (start != null && Std.isOfType(start, VariantType)) {
                try { // some neko strangness here with exception being thrown on the switch
                    switch (start) {
                        case VariantType.VT_String(v):
                            start = v;
                            isVariant = true;
                        case _:
                    }
                } catch (e:Dynamic) { }
            }

In rare circumstances, the innermost block of code will be accessed, despite start not being a VT_String value or even being of the type VariantType. If it reaches this line, the expression start = v will attempt to access the value v, which does not exist, causing an access violation and immediately crashing the app without throwing a Haxe exception.

This results in the following issue, which this PR reliably resolves: FunkinCrew/Funkin#5011

In the above issue, the crash happens when target is a Tooltip, componentProperty is "opacity", and start is an integer value of 1.