vo_gpu_next: disable auto reference white on onlder libplacebo version

To avoid forcing by default values that may be problematic in some
cases, especially with ICC profiles.

Author: kasper93

DOCS/man/options.rst

@@ -7542,7 +7542,8 @@ them.
 
     In ``auto`` mode (default), the reference white luminance is queried from
     the system. This is currently only supported on Windows. If the system does
-    not provide a value, 203 cd/m² is assumed.
+    not provide a value, 203 cd/m² is assumed. (``auto`` mode works only
+    with libplacebo >= 371)
 
     .. note::
 

video/out/vo_gpu_next.c

@@ -775,10 +775,15 @@ static float get_ref_luma(struct priv *p)
     if (opts->hdr_reference_white)
         return opts->hdr_reference_white;
 
+#if PL_API_VER >= 371
     // auto: follow the system reference white, if available
+    // Use auto mode only on libplacebo >= 371, to avoid luminance mismatches,
+    // on SDR->SDR in some corner cases. User override is still respected, to
+    // preserve previous behavior.
     struct ra_swapchain *sw = p->ra_ctx->swapchain;
     if (sw->fns->target_ref_luma)
         return sw->fns->target_ref_luma(sw);
+#endif
 
     return 0;
 }