Allow Display.text_pixels() to receive a loaded font or a font string. (

#601)

* Allow Display.text_pixels() to receive a loaded font or a font string.

This will allow the users to preload a font and eliminate the constant
reloading of fonts, which can produce lag when displaying in a loop.

* doc-string for Display.text_pixels and Display.text_grid updated for sphinx.

ev3dev2/display.py

@@ -365,6 +365,7 @@ def text_pixels(self, text, clear_screen=True, x=0, y=0, text_color='black', fon
         Display `text` starting at pixel (x, y).
 
         The EV3 display is 178x128 pixels
+
         - (0, 0) would be the top left corner of the display
         - (89, 64) would be right in the middle of the display
 
@@ -374,15 +375,22 @@ def text_pixels(self, text, clear_screen=True, x=0, y=0, text_color='black', fon
         https://www.w3schools.com/colors/colors_names.asp
 
         'font' : can be any font displayed here
-            http://ev3dev-lang.readthedocs.io/projects/python-ev3dev/en/ev3dev-stretch/other.html#bitmap-fonts
+            http://ev3dev-lang.readthedocs.io/projects/python-ev3dev/en/ev3dev-stretch/display.html#bitmap-fonts
+
+        - If font is a string, it is the name of a font to be loaded.
+        - If font is a Font object, returned from :meth:`ev3dev2.fonts.load`, then it is
+          used directly.  This is desirable for faster display times.
+
         """
 
         if clear_screen:
             self.clear()
 
         if font is not None:
-            assert font in fonts.available(), "%s is an invalid font" % font
-            return self.draw.text((x, y), text, fill=text_color, font=fonts.load(font))
+            if isinstance(font, str):
+                assert font in fonts.available(), "%s is an invalid font" % font
+                font = fonts.load(font)
+            return self.draw.text((x, y), text, fill=text_color, font=font)
         else:
             return self.draw.text((x, y), text, fill=text_color)
 
@@ -400,7 +408,12 @@ def text_grid(self, text, clear_screen=True, x=0, y=0, text_color='black', font=
         https://www.w3schools.com/colors/colors_names.asp
 
         'font' : can be any font displayed here
-            http://ev3dev-lang.readthedocs.io/projects/python-ev3dev/en/ev3dev-stretch/other.html#bitmap-fonts
+            http://ev3dev-lang.readthedocs.io/projects/python-ev3dev/en/ev3dev-stretch/display.html#bitmap-fonts
+
+        - If font is a string, it is the name of a font to be loaded.
+        - If font is a Font object, returned from :meth:`ev3dev2.fonts.load`, then it is
+          used directly.  This is desirable for faster display times.
+
         """
 
         assert 0 <= x < Display.GRID_COLUMNS,\