Gaphics: Make frame skip a config option

When using something like Valgrind that will run
mkxp 20 times slower than normal, frame skip will
make the redraw loop completely grind to a halt.
Set 'frameSkip' to false in the config to
avert this.

src/config.cpp

@@ -38,6 +38,7 @@ Config::Config()
       defScreenW(640),
       defScreenH(480),
       fixedFramerate(0),
+      frameSkip(true),
       solidFonts(false),
       gameFolder("."),
       allowSymlinks(false)
@@ -58,6 +59,7 @@ void Config::read()
 	READ_VAL(defScreenW, Int);
 	READ_VAL(defScreenH, Int);
 	READ_VAL(fixedFramerate, Int);
+	READ_VAL(frameSkip, Bool);
 	READ_VAL(solidFonts,  Bool);
 	READ_VAL(gameFolder, ByteArray);
 	READ_VAL(allowSymlinks, Bool);