lavfi/vf_libplacebo: test for renderable pixel formats

Requires a new upstream function to test not for *import* support on a
given output pixel format, but also whether we can render to it.

Fixes: https://github.com/haasn/libplacebo/issues/173

libavfilter/vf_libplacebo.c

@@ -1141,6 +1141,11 @@ static int libplacebo_query_format(AVFilterContext *ctx)
             }
         }
 
+#if PL_API_VER >= 293
+        if (!pl_test_pixfmt_caps(s->gpu, pixfmt, PL_FMT_CAP_RENDERABLE))
+            continue;
+#endif
+
         RET(ff_add_format(&outfmts, pixfmt));
     }