Cleanup on debug callback.

[asuessenbach]

Description

The debug callback is called with just one severity bit, depending on the MessageSeverity bits used on creation.
There might be multiple bits in the MessageType argument.
That is, the message_type, as well as the callback_data are used in the generated message, while the message_severity determines the kind of logging (debug/info/warning/error).

Should any additional member of the callback_data be added to the message?

If this approach is considered to be good, I'd carry it over to all the other occurances of a debug callback.

Build tested on Win11 with VS2026. Run tested on Win11 with NVidia GPU.

Fixes #1558

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