Replace OSystem::hasAlpha with a feature flag

svn-id: r17695

backends/dc/dc.h

@@ -133,7 +133,6 @@ class OSystem_Dreamcast : public OSystem {
   void clearOverlay();
   void grabOverlay(int16 *buf, int pitch);
   void copyRectToOverlay(const int16 *buf, int pitch, int x, int y, int w, int h);
-  bool hasAlpha() const { return true; }
   OverlayColor RGBToColor(uint8 r, uint8 g, uint8 b) { return ARGBToColor(255, r, g, b); }
   void colorToRGB(OverlayColor color, uint8 &r, uint8 &g, uint8 &b) {
     uint8 tmp; colorToARGB(color, tmp, r, g, b);

backends/dc/dcmain.cpp

@@ -151,6 +151,7 @@ bool OSystem_Dreamcast::hasFeature(Feature f)
   switch(f) {
   case kFeatureAspectRatioCorrection:
   case kFeatureVirtualKeyboard:
+  case kFeatureOverlaySupportsAlpha:
     return true;
   default:
     return false;

common/system.h

@@ -110,7 +110,12 @@ public:
 		 * Entertainment games. If backend doesn't implement this feature then
 		 * engine switches to b/w version of cursors.
 		 */
-		kFeatureCursorHasPalette
+		kFeatureCursorHasPalette,
+
+		/**
+		 * Set to true if the overlay pixel format has an alpha channel.
+		 */
+		kFeatureOverlaySupportsAlpha
 	};
 	
 	/**
@@ -460,13 +465,6 @@ public:
 	virtual int overlayToScreenX(int x) { return x; }
 	virtual int overlayToScreenY(int y) { return y; }
 
-	/**
-	 * Return true if the overlay pixel format has an alpha channel.
-	 */
-	virtual bool hasAlpha() const {
-		return false;
-	}
-
 	/**
 	* Convert the given RGB triplet into an OverlayColor. A OverlayColor can
 	 * be 8bit, 16bit or 32bit, depending on the target system. The default

gui/newgui.cpp

@@ -365,7 +365,7 @@ void NewGui::blendRect(int x, int y, int w, int h, OverlayColor color, int level
 	if (!rect.isValidRect())
 		return;
 
-	if (_system->hasAlpha()) {
+	if (_system->hasFeature(OSystem::kFeatureOverlaySupportsAlpha)) {
 
 		int a, r, g, b;
 		uint8 aa, ar, ag, ab;