st/mesa: comments in update_viewport()

src/mesa/state_tracker/st_atom_viewport.c

@@ -47,10 +47,14 @@ update_viewport( struct st_context *st )
    /* _NEW_BUFFERS
     */
    if (st_fb_orientation(ctx->DrawBuffer) == Y_0_TOP) {
+      /* Drawing to a window.  The corresponding gallium surface uses
+       * Y=0=TOP but OpenGL is Y=0=BOTTOM.  So we need to invert the viewport.
+       */
       yScale = -1;
       yBias = (GLfloat)ctx->DrawBuffer->Height;
    }
    else {
+      /* Drawing to an FBO where Y=0=BOTTOM, like OpenGL - don't invert */
       yScale = 1.0;
       yBias = 0.0;
    }