Have presentations request their own full-screen clears

This will let custom ship spins use Presentations to
display data over themselves without wiping out the
underlyin image.

git-svn-id: svn://svn.code.sf.net/p/sc2/code/trunk@2845 8092fc87-c524-0410-9efc-e669fe64eaf9

sc2/src/sc2code/intro.c

@@ -730,10 +730,6 @@ ShowPresentation (STRING PresStr)
 	OldContext = SetContext (ScreenContext);
 	GetContextClipRect (&OldRect);
 	OldFont = SetContextFont (NULL);
-	/* paint black rect over screen	*/
-	SetContextBackGroundColor (
-			BUILD_COLOR (MAKE_RGB15 (0x00, 0x00, 0x00), 0x00));
-	ClearDrawable ();
 	UnlockMutex (GraphicsLock);
 
 	FlushInput ();