Add uView.screenBuffer() for access to screen RAM.

Returns a pointer the the start of the RAM screen buffer
for direct write and read access.

MicroView.h

@@ -192,6 +192,7 @@ public:
 	uint8_t getLCDHeight(void);
 	void setColor(uint8_t color);
 	void setDrawMode(uint8_t mode);
+	uint8_t *screenBuffer(void);
 
 	// Font functions
 	uint8_t getFontWidth(void);