Cppia debugger reads a function's variables from the wrong frame base #248

Upstream: HaxeFoundation/hxcpp#1426 by @robknopf (open PR).

With HXCPP_STACK_SCRIPTABLE, the debugger reads a cppia function's this and locals from inStack - stackSize in ScriptCallable::getScriptableVariables, getScriptableValue and setScriptableValue (src/hx/cppia/CppiaFunction.cpp). That isn't where the frame starts: a call sets ctx->frame before this and the arguments are pushed, the function's StackLayout already counts them, and runFunction then allocates the whole layout again before CPPIA_STACK_FRAME pushes the hx::StackFrame. So the frame sits at frame + sizeof(hx::Object *) + argBytes + stackSize, and inStack - stackSize lands on the first argument (or the first local), not on this. The debugger then shows and sets the wrong variables.

libhxcpp still has the three inStack - stackSize reads. Port the upstream fix with a debugger test that reads this, an argument and a local of a cppia function.