corelib: improved default palette

- light gray instead of 2x dark gray
- add palette file for paint programs
- new sprite with default palette for graphbench

examples/graphbench.pas

@@ -106,7 +106,7 @@ begin
 end;
 
 begin
-	readSpriteData('rocket.sprt');
+	readSpriteData('sprite-testcard.sprt');
 
 	InitGraphics;
 	startBench('points 200K');