[ODE] capturing individual frames

[Alon Levy]

please add this to the code. It's a small addition to drawstuff to enable capturing of
single frames, and with that the user can do whatever, like capturing every 5 frames instead of
every frame.

I don't know how to use diff, so here is the code. There are 3 files to change: windows.cpp, x11.cpp
and drawstuff.h .

* in windows.cpp (no support for capturing, so this is just to let the program compile smoothly in
  windows and in x11)

\\\\ paste after: static HWND main_window = 0;

static int writeframes=0;		// 1 if frame files to be written

static void setWriteFrames() { writeframes = 1; };
static void clearWriteFrames() { writeframes = 0; };
static void toggleWriteFrames() { writeframes ^= 1; };

* in x11.cpp

\\\\\ paste after: static in writeframes=0;

static void setWriteFrames() { writeframes = 1; };
static void clearWriteFrames() { writeframes = 0; };
static void toggleWriteFrames() { writeframes ^= 1; };

* in drawstuff.h (add at the end)

void setWriteFrames();
void clearWriteFrames();
void toggleWriteFrames();