This fixes numerous problems regarding dead keys on Wayland. Most notably, Wayland was enforcing dead keys on SDL_KEYDOWN and SDL_KEYUP events, which caused unresponsiveness on keys that were mapped to dead keys (tilde on US-Intl is most notable for this, commonly used as a console key).
When starting text input, not all state was reset properly. The text input protocol requires to be re-enabled every time text input changes, which SDL did not do. Also, XKB compose state was not reset at all, causing composite and dead keys to carry over from when text input was disabled.
Manual cherry-pick of 1c3090a1ac by Hanicef
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