vo_gpu: mark the ra_ctx type in the ra_ctx.log

This is useful since it makes backend messages/errors get annotated
with either "opengl" or "vulkan". The nice thing about doing the log
transition here is that all of the platform-neutral code in the rest of
vo_gpu doesn't inherit it, but everything concerning the actual context,
spir-v compiler, RA backend, etc. does.

video/out/gpu/context.c

@@ -132,7 +132,7 @@ struct ra_ctx *ra_ctx_create(struct vo *vo, enum ra_api_type type,
         *ctx = (struct ra_ctx) {
             .vo = vo,
             .global = vo->global,
-            .log = vo->log,
+            .log = mp_log_new(ctx, vo->log, type_names[contexts[i]->type]),
             .opts = opts,
             .fns = contexts[i],
         };