Fix label background_color setter

adafruit_display_text/__init__.py

@@ -279,15 +279,13 @@ def background_color(self):
         """Color of the background as an RGB hex number."""
         return self._background_color
 
+    def _set_background_color(self, new_color):
+        # subclasses should override this
+        pass
+
     @background_color.setter
     def background_color(self, new_color):
-        self._background_color = new_color
-        if new_color is not None:
-            self.palette[0] = new_color
-            self.palette.make_opaque(0)
-        else:
-            self.palette[0] = 0
-            self.palette.make_transparent(0)
+        self._set_background_color(new_color)
 
     @property
     def anchor_point(self):

adafruit_display_text/bitmap_label.py

@@ -554,3 +554,12 @@ def _set_font(self, new_font):
     def _set_text(self, new_text, scale):
         new_text = self._tab_text.join(new_text.split("\t"))
         self._reset_text(text=new_text, scale=self.scale)
+
+    def _set_background_color(self, new_color):
+        self._background_color = new_color
+        if new_color is not None:
+            self.palette[0] = new_color
+            self.palette.make_opaque(0)
+        else:
+            self.palette[0] = 0
+            self.palette.make_transparent(0)

adafruit_display_text/label.py

@@ -329,3 +329,6 @@ def _set_line_spacing(self, new_line_spacing):
 
     def _set_text(self, new_text, scale):
         self._reset_text(new_text)
+
+    def _set_background_color(self, new_color):
+        self._update_background_color(new_color)