Shaders: Add error message for asciiart shader if shader_subgroup is missing.

This extension is currently required to get good performance.
So instead of maybe triggering a GPU hang, let's just disable the shader and show a warning instead.

Sadly our postprocessing framework does not support reporting a warning, so the way to display the warning is a bit hacky.
But this is an ascii-art shader, we already have a font in the shader, so let's use it.

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