reduce View Palette lag

[Asuna Yuuki Nagato]

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:

		// 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;
		}