adding height and width to OnDiskBitmap for #1460

[siddacious]

I added height and width to OnDiskBitmap as per #1460

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Adafruit CircuitPython 4.0.0-alpha.5-168-gd2a7cd6c5-dirty on 2019-01-20; Adafruit Trellis M4 Express with samd51g19
>>> from displayio import OnDiskBitmap
>>> f = open("1x20orange.bmp", "rb")
>>> odb = OnDiskBitmap(f)
>>> odb.height
20
>>> odb.width
1
>>>

[tannewt]

Woohoo! Great job! Thank you!