Image needs addititonal attrs

[FoamyGuy]

Originally noted here: adafruit/Adafruit_CircuitPython_RGB_Display#111 (comment)

The Image interface seems incomplete. With the changed import:

adafruit_rgb_display/rgb.py:68: error: "Image" has no attribute "convert" [attr-defined]
adafruit_rgb_display/rgb.py:213: error: "Image" has no attribute "rotate" [attr-defined]
adafruit_rgb_display/rgb.py:228: error: "Image" has no attribute "getpixel" [attr-defined]

I will still commit the change.

Image needs to have convert, rotate, and getpixel to satisfy those errors.