imguigml_begin_combo fix

#define imguigml_begin_combo
///@function imguigml_begin_combo(_label, _preview_value, [_flags=0])
///@desc begin a combo widget
///@param {String} _label
///@param {String} _preview_val
///@param {Real:EImGui_ComboFlags} [_flags=0]
///@returns {Bool}
var _label = argument[0],
    _preview_val = argument[1],
        _flags = argument_count > 2 ? argument[2] : 0;

var in = __Imgui_in;
sr_buffer_write_val(in, _label,       ERousrData.String);
sr_buffer_write_val(in, _preview_val, ERousrData.String);
sr_buffer_write_val(in, _flags,       ERousrData.Uint32);

// Add these lines \/\/\/
__imguigml_ext_call(_extImguiGML_begin_combo());
var out = __Imgui_out;
var _ret = buffer_read(out, buffer_s8) == 1;
return _ret;