vo_opengl: describe the remainder passes after user shaders

On optional hook points, we store to a temp FBO and then read from it
again to complete any operations that may still be left (e.g.
sigmoidization after MAIN/LINEAR).

In theory this mechanism should be reworked to avoid the temporary FBO
until the next time we actually need one - and also skip redundant
passes if we the next thing we need *is* a FBO - but both are those are
tricky. Anyway, in the meantime, at least we can label the
(semi-)redundant passes that get generated when using user shaders.

video/out/opengl/video.c

@@ -1356,6 +1356,7 @@ found:
     p->texture_w = img.w;
     p->texture_h = img.h;
     p->components = img.components;
+    pass_describe(p, "(remainder pass)");
 }
 
 static void load_shader(struct gl_video *p, struct bstr body)