[Ogre 1.x] Unloading engine plugin causes tests to crash on Windows

**Original report ([archived issue](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-rendering/issues/45)) by Ian Chen (Bitbucket: [Ian Chen](https://bitbucket.org/%7Beaa6fca5-6deb-43f6-907f-971c144735dd%7D/), GitHub: [iche033](https://github.com/iche033)).**

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# Description

On windows, the tests run fine and calls to [unloadEnginePlugin](https://github.com/ignitionrobotics/ign-rendering/blob/f4749d4e98cf2b654fcbab8b1c42a4e91ed9338a/src/RenderEngineManager.cc#L397) return without an error but the tests crash on exit with the following msg

```
unknown file
SEH exception with code 0xc0000005 thrown in the test body.
```
Note that this problem is observed on our windows jenkins machine that currently only run tests using the `ogre` rendering engine

# Steps to Reproduce

1. Run any ignition rendering test

**Expected behavior:**

Test passes

**Actual behavior:**

Test crashes with error msg

**Reproduces how often:**

All the time

# Versions

All versions