Bump deps/sdl from e9535fa to d15ecb2

Bumps [deps/sdl](https://github.com/libsdl-org/SDL) from `e9535fa` to `d15ecb2`.
- [Release notes](https://github.com/libsdl-org/SDL/releases)
- [Commits](libsdl-org/SDL@e9535fa...d15ecb2)

---
updated-dependencies:
- dependency-name: deps/sdl
  dependency-version: d15ecb21bead102489cb2180ac48a43dfe2e2676
  dependency-type: direct:production
...

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Author: dependabot[bot]