`win32_filesystem_monitor` pumps 1 event per handle on `pump` calls

### Checklist

- [x] I did not find a duplicate of this bug in the Github Issues section.

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### Description

In `extras/win32-bindings`, the `win32_filesystem_monitor` only pumps a single event per watched handle during an invocation of `pump`, rather than pumping _all_ events at once.

This may be a larger issue for system that would like to watch larger amounts of files at once, since I/O events are typically polled once per update cycle and/or frame.

### Steps to reproduce

1. Create a `win32_filesystem_monitor`
2. Register the monitor to a `message_pump`
3. Register a listener to the `message_pump`
4. `watch` a directory
5. Trigger several filesystem actions
6. Invoke `message_pump::pump`

#### Expected Behavior

All events fire

#### Actual Behavior

A single event per watched directory fires per `pump` invocation

### Extra information

* Operating System: **Windows 10**
* Compiler: **clang-8.0**