Draft: backends/native: Introduce HDR_OUTPUT_METADATA and set it to SDR

No functional change but this makes sure the EOTF is always set to SDR even if other KMS applications mess with it.

HDR10 mode can be force enabled with FORCE_HDR10_ENABLE=1.

Depends on !2379 (merged) !2385 (closed) and !2351 (merged)