union method
Implementation
math.Rectangle<double> union(math.Rectangle<double> b) {
var aPointer = calloc();
var bPointer = b.calloc();
var resultPointer = ffi.calloc<SdlRect>();
aPointer.unionRect(bPointer, resultPointer);
var result = resultPointer.create();
ffi.calloc.free(aPointer);
ffi.calloc.free(bPointer);
ffi.calloc.free(resultPointer);
return result;
}