mode7 doc: update screenshots to be png files

chiptune_debug/NOTES

@@ -17,3 +17,5 @@ remove multi file (down to 973) still issue
 remove dos33 code 		still issue
 remove lz4 code			still issue
 remove interrupt driven		still issue!?
+play at 25Hz			still issue
+

mode7_demo/docs/mode7_demo.tex

@@ -225,28 +225,28 @@ the actual Apple II (an Apple IIe platinum edition).
 
 \begin{figure}[tb]
 \begin{center}
-\includegraphics[width=\columnwidth]{figures/m7_screen1.jpg}
+\includegraphics[width=\columnwidth]{figures/m7_screen1.png}
 \caption{Bouncing ball on infinite checkerboard.\label{fig:ball}}
 \end{center}
 \end{figure}
 
 \begin{figure}[tb]
 \begin{center}
-\includegraphics[width=\columnwidth]{figures/m7_screen4.jpg}
+\includegraphics[width=\columnwidth]{figures/m7_screen4.png}
 \caption{Spaceship flying over an island.\label{fig:tb1}}
 \end{center}
 \end{figure}
 
 \begin{figure}[tb]
 \begin{center}
-\includegraphics[width=\columnwidth]{figures/m7_screen3.jpg}
+\includegraphics[width=\columnwidth]{figures/m7_screen3.png}
 \end{center}
 \caption{Spaceship with starfield.\label{fig:stars}}
 \end{figure}
 
 \begin{figure}[tb]
 \begin{center}
-\includegraphics[width=\columnwidth]{figures/m7_screen2.jpg}
+\includegraphics[width=\columnwidth]{figures/m7_screen2.png}
 \end{center}
 \caption{Rasterbars, stars, and credits.  Stealth Susie was a particularly
 well-traveled guinea pig.