repi

Right now it is vendored under `fsr-sys/FidelityFX-FSR2`, but would be easier to update, and to make sure we don't do repo-specific changes in it, if it is a git submodule...

```sh $ cargo test ---- fsr/src/lib.rs - (line 5) stdout ---- error: expected one of `!`, `,`, `.`, `::`, `?`, `{`, `}`, or an operator, found `message_callback` --> fsr/src/lib.rs:25:5 |...

These functions do not have `# Safety` documentation to describe how to uphold safety invariants when calling them, clippy finds it: ``` warning: unsafe function's docs miss `# Safety` section...

Was building on Linux and get the following compiler warnings outputted that we should fix, or most likely suppress in the build script. ``` warning: FidelityFX-FSR2/src/ffx-fsr2-api/ffx_fsr2.cpp:190:63: warning: missing initializer for...