fbaccel: implement partial blitting for SPARK

Blit only the parts of the screen that have changed. First
tests show better performance and no obvious errors. Might
still contain some bugs, you have been warned :-)

src/driver/fbaccel.h

@@ -36,6 +36,10 @@
 #include <OpenThreads/Thread>
 #include <OpenThreads/Condition>
 
+#if HAVE_SPARK_HARDWARE
+#define PARTIAL_BLIT 1
+#endif
+
 class CFrameBuffer;
 class CFbAccel
 	: public OpenThreads::Thread
@@ -57,6 +61,16 @@ class CFbAccel
 		bool blit_pending;
 		OpenThreads::Condition blit_cond;
 		OpenThreads::Mutex blit_mutex;
+#ifdef PARTIAL_BLIT
+		OpenThreads::Mutex to_blit_mutex;
+		struct {
+			int xs;
+			int ys;
+			int xe;
+			int ye;
+		} to_blit;
+		uint32_t last_xres;
+#endif
 	public:
 		fb_pixel_t *backbuffer;
 		fb_pixel_t *lbb;