fix various minor issues causing compilation failure with clang (#567)

- Include order and forward decls were breaking button.h because a
declared but not defined struct can't be initiated (honestly this file
was cooked, crazy include order).
- `MAXINT` is GCC specific, `INT_MAX` is portable.
- InterlockedDecrement takes a LONG, not ULONG
- an imgui printf-style call using a direct string instead of %s (let's
ignore the fact that it's actually safe based on how the str is
constructed)
- missing `override` on a virtual subclass
- `CALLBACK` on a lambda threw me for a loop, but I believe moving it
after the arg list is the correct approach (see if it builds on gcc I
guess)
- `std::result_of` was removed in C++20, which the project is built with

src/spice2x/overlay/windows/iopanel.h

@@ -2,6 +2,7 @@
 
 #include "overlay/window.h"
 #include "cfg/button.h"
+#include "cfg/light.h"
 
 namespace overlay::windows {