Adds the necessary routine for all machines to be able to respond to get_scan_status.

They all just as the CRT, as all are currently based on the CRT. Which doesn't currently know the total clock rate it would need to in order properly to scale the answer to the question. Further thought coming.

Components/9918/9918.cpp

@@ -117,6 +117,10 @@ void TMS9918::set_scan_target(Outputs::Display::ScanTarget *scan_target) {
 	crt_.set_scan_target(scan_target);
 }
 
+Outputs::Display::ScanStatus TMS9918::get_scan_status() const {
+	return crt_.get_scan_status();
+}
+
 void TMS9918::set_display_type(Outputs::Display::DisplayType display_type) {
 	crt_.set_display_type(display_type);
 }

Components/9918/9918.hpp

@@ -44,6 +44,9 @@ class TMS9918: public Base {
 		/*! Sets the scan target this TMS will post content to. */
 		void set_scan_target(Outputs::Display::ScanTarget *);
 
+		/// Gets the current scan status.
+		Outputs::Display::ScanStatus get_scan_status() const;
+
 		/*! Sets the type of display the CRT will request. */
 		void set_display_type(Outputs::Display::DisplayType);