reduce View Palette lag

Mousing over the View->View Palette window feels laggy because of how infrequently the palette number indicator left of the OK button is redrawn.

Lowering this rvalue in gui.cpp makes it more responsive:

Code:

---

                // Not perfect, but beats using 100% of CPU.
                // The above may miss things, so draw at least a few times a second.
                if (num_idle_frames++ == 15)
                {
                        should_draw = 1;
                }

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