Added a Refresh button to ImageReaders

Don't feel obliged to change this immediately, but for these kind of random arguments which are unobvious, I'm trying to adopt a style of `( /* force = */ true )` to document what the parameter is with a pseudo-named-argument. I'm finding it quite handy when coming back to code I haven't looked at for a while. There's also a proposal for named arguments in C++ which we could migrate the pseudo ones to easily if it gets adopted.

Did you give any thought to not forcing everything to be invalidated, but instead letting OIIO decide based on modification times? In theory that might be nice because we wouldn't be invalidating the cache for other nodes as well. Are there use cases where we need a refresh but modification time hasn't changed?