fb_accel_td: add missing getBackBufferPointer()

this fixes tuxtxt rendering problems

src/driver/fb_accel.h

@@ -171,6 +171,7 @@ class CFbAccelTD
 		void paintLine(int xa, int ya, int xb, int yb, const fb_pixel_t col);
 		void blit2FB(void *fbbuff, uint32_t width, uint32_t height, uint32_t xoff, uint32_t yoff, uint32_t xp, uint32_t yp, bool transp);
 		void waitForIdle(const char *func = NULL);
+		fb_pixel_t * getBackBufferPointer() const;
 		void setBlendMode(uint8_t);
 		void setBlendLevel(int);
 };

src/driver/fb_accel_td.cpp

@@ -202,3 +202,8 @@ void CFbAccelTD::setBlendLevel(int level)
 	if (level == 100) // sucks
 		usleep(20000);
 }
+
+fb_pixel_t *CFbAccelTD::getBackBufferPointer() const
+{
+	return backbuffer;
+}