Make clearScene() set the _needFullRedraw variable.

I think the reason I didn't do this from the start was that BS2 used to
call clearScene(), or whatever the function was called back then, between
every frame. Nowadays, it simply assumes that each frame will cover the
previous one.

Anyway, this change prevents the restart/restore dialog from appearing
briefly between the two intro cutscene animations.

svn-id: r13421

sword2/driver/d_draw.cpp

@@ -115,6 +115,7 @@ void Graphics::setRenderLevel(int8 level) {
 
 void Graphics::clearScene(void) {
 	memset(_buffer + MENUDEEP * _screenWide, 0, _screenWide * RENDERDEEP);
+	_needFullRedraw = true;
 }
 
 } // End of namespace Sword2