clean up detection and handling of drivers with problematic AA lines

only draw topmost screen if topmost screen isn't transparent (speeds up options menus in editor)
optimized beat bar drawing in NoteField

stepmania/src/ScreenPrompt.cpp

@@ -28,6 +28,8 @@ const float PROMPT_Y	=	CENTER_Y + 120;
 
 ScreenPrompt::ScreenPrompt( ScreenMessage SM_SendWhenDone, CString sText, bool bYesNoPrompt, bool bDefaultAnswer, void(*OnYes)(), void(*OnNo)() )
 {
+	m_bIsTransparent = true;	// draw screens below us
+
 	m_SMSendWhenDone = SM_SendWhenDone;
 	m_bYesNoPrompt = bYesNoPrompt;
 	m_bAnswer = bDefaultAnswer;