(Qt) Shader menu can freeze RetroArch on closing content/exiting

[LazyBumHorse]

Description

Closing content or exiting RetroArch when the Qt shader menu is open can cause crashes, freezes and sometimes hundreds of messages like:

QBasicTimer::start: QBasicTimer can only be used with threads started with QThread
QBasicTimer::start: QBasicTimer can only be used with threads started with QThread
QBasicTimer::start: QBasicTimer can only be used with threads started with QThread
...
QObject::killTimer: Timers cannot be stopped from another thread
QObject::~QObject: Timers cannot be stopped from another thread

This seems to be related to the way shader parameters are loaded, as the crash seems to happen somewhere after

[INFO] [Qt]: Reloading shader parameters.

Sometimes this is the very last log message.

Steps to reproduce the bug

The best way I found to reproduce:

1. Load shaders_glsl/crt/crt-royale.glslp
2. Open Qt shader menu
3. Close Content or Exit to freeze and eventually crash RetroArch

It might not always crash, but with crt-royale there seems to be a very high probability.

Bisect Results

I noticed this issue while testing for #9315 (but couldn't find a good way to reproduce it). It was on master before that and it might be an issue that is as old as the Qt shader menu itself.