Loading .fx files in MonoGame 3.8.3 and newer crashes due to absolute paths

[vchelaru]

If a .fx file is loaded from disk and the project targets MonoGame 3.8.3 or newer, the project crashes. This is a regression in MonoGame 3.8.3 as outlined here:

MonoGame/MonoGame#3165

We can solve this one of two ways:

1. See if MonoGame is willing to make the fix so we don't have to change our code (preferred)
2. Modify FlatRedBall source to pass in relative paths IF the file being loaded is relative to the .exe. This adds a little complexity and I'd rather not do that. This doesn't address issues of loading files temporarily from other locations which might happen in tools or live edit.