Codechange: Allow blitter factories to decide at runtime if the blitter is usable.

src/blitter/factory.hpp

@@ -73,6 +73,15 @@ protected:
 		}
 	}
 
+	/**
+	 * Is the blitter usable with the current drivers and hardware config?
+	 * @return True if the blitter can be instantiated.
+	 */
+	virtual bool IsUsable() const
+	{
+		return true;
+	}
+
 public:
 	virtual ~BlitterFactory()
 	{
@@ -119,7 +128,7 @@ public:
 		for (; it != GetBlitters().end(); it++) {
 			BlitterFactory *b = (*it).second;
 			if (strcasecmp(bname, b->name.c_str()) == 0) {
-				return b;
+				return b->IsUsable() ? b : nullptr;
 			}
 		}
 		return nullptr;