let Actor subclasses have first crack at parsing commands

allow Models and BitmapTexts in BGAnimationLayer (detected through file extension)
add RageModelVertex, which doesn't have a color property
add "stretch" file name hint for textures

stepmania/src/BitmapText.cpp

@@ -178,7 +178,7 @@ void BitmapText::BuildChars()
 
 		for( unsigned j=0; j<szLine.size(); j++ )	// for each character in the line
 		{
-			RageVertex v[4];
+			RageSpriteVertex v[4];
 			const glyph &g = m_pFont->GetGlyph(szLine[j]);
 
 			/* set vertex positions */