Add XBitmap (*.xbm) support

add XBitmap(*.xbm) support, to directly use exported GIMP xbm files. (Rename the file to *.c and open in editor.)

Adafruit_GFX.h

@@ -48,6 +48,8 @@ class Adafruit_GFX : public Print {
       int16_t radius, uint16_t color),
     drawBitmap(int16_t x, int16_t y, const uint8_t *bitmap,
       int16_t w, int16_t h, uint16_t color),
+    drawXBitmap(int16_t x, int16_t y, const uint8_t *bitmap, 
+      int16_t w, int16_t h, uint16_t color),
     drawChar(int16_t x, int16_t y, unsigned char c, uint16_t color,
       uint16_t bg, uint8_t size),
     setCursor(int16_t x, int16_t y),