vo_opengl: add log field to MGLContext

Should have done this 1000 years ago. Now GL backends can use mp_log
macros directly on the MPGLContext, instead of doing stupid things like
for example MP_WARN(ctx->vo, ...).

video/out/opengl/context.c

@@ -160,6 +160,7 @@ static MPGLContext *init_backend(struct vo *vo, const struct mpgl_driver *driver
         .vo = vo,
         .global = vo->global,
         .driver = driver,
+        .log = vo->log,
     };
     if (probing)
         vo_flags |= VOFLAG_PROBING;

video/out/opengl/context.h

@@ -78,6 +78,7 @@ typedef struct MPGLContext {
     struct vo *vo;
     const struct mpgl_driver *driver;
     struct mpv_global *global;
+    struct mp_log *log;
 
     // For hwdec_vaegl.c.
     const char *native_display_type;