mesa: fix renderbuffer leak

We don't need to call _mesa_reference_renderbuffer() for the first
assignment as refCount starts at 1. For swrast we work around the
fact we will indirectly call _mesa_reference_renderbuffer() by
resetting refCount to 0.

Fixes: 32141e53d1 (mesa: tidy up renderbuffer RefCount initialisation)

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>

src/mesa/main/fbobject.c

@@ -451,7 +451,7 @@ _mesa_update_texture_renderbuffer(struct gl_context *ctx,
          _mesa_error(ctx, GL_OUT_OF_MEMORY, "glFramebufferTexture()");
          return;
       }
-      _mesa_reference_renderbuffer(&att->Renderbuffer, rb);
+      att->Renderbuffer = rb;
 
       /* This can't get called on a texture renderbuffer, so set it to NULL
        * for clarity compared to user renderbuffers.

src/mesa/swrast/s_renderbuffer.c

@@ -271,6 +271,11 @@ add_color_renderbuffers(struct gl_context *ctx, struct gl_framebuffer *fb,
          return GL_FALSE;
       }
 
+      /* Set refcount to 0 to avoid a leak since the _mesa_add_renderbuffer()
+       * call below will bump the initial refcount.
+       */
+      rb->RefCount = 0;
+
       rb->InternalFormat = GL_RGBA;
 
       rb->AllocStorage = soft_renderbuffer_storage;