Fix Clyde deadlock on nvidia/wayland

[DamianX]

I am not well-versed in OpenGL but I think this is correct.
According to the docs here:

If a sync object is associated with a fence command issued previously, but not yet flushed, ClientWaitSync may hang forever.

We use WaitSync instead of ClientWaitSync, so I'm not sure how relevant that is, however the docs also say:

Applications which block on a fence sync object must take additional steps to assure that the context from which the corresponding fence command was issued has flushed that command.

In our case, we do FenceSync here:

RobustToolbox/Robust.Client/Graphics/Clyde/Clyde.Rendering.cs

Line 1171 in 1d0b581

rt.LastGLSync = GL.FenceSync(SyncCondition.SyncGpuCommandsComplete, WaitSyncFlags.None);

and then we WaitSync on it here:

RobustToolbox/Robust.Client/Graphics/Clyde/GLContext/GLContextWindow.cs

Line 206 in 1d0b581

GL.WaitSync(sync, WaitSyncFlags.None, unchecked((long) 0xFFFFFFFFFFFFFFFFUL));

And we don't Flush() at any point.

Additionally, Issue 20 in the docs also indicates a Flush() after FenceSync() is needed in this situation, which is what my change adds. It does still mention ClientWaitSync instead of WaitSync, and I'm not sure if that matters.

This change does fix the issue on my end so it closes #6958