unix_sdl: fix input polling overhead

[ChrisDeadman]

Summary

Fix SDL input polling overhead by using SDL_WaitEventTimeout instead of pure SDL_PollEvent loop.
This removes a hot path in SDL backend.

Checklist

• Closes N/A
• I have tested my changes locally and validated that the functionality works as intended
• I have discussed this with core contributors already
• This pull request doesn't require changes to the ROM set
• This pull request doesn't require changes to the asset set