M5EPD_Canvas::setDriver does not initialise correctly

[murraypaul]

M5EPD_Canvas::M5EPD_Canvas(M5EPD_Driver *driver) set _bpp to 4, overriding the default value of 16 from TFT_eSprite.

Neither M5EPD_Canvas::M5EPD_Canvas() nor M5EPD_Canvas::setDriver set _bpp.

(They also do not set _bytewidth, _last_push_x or _last_push_y, leaving them uninitialised.)

That means that

M5EPD_Canvas Canvas;
Canvas.setDriver(&M5.EPD);

Does not work in the same way that

M5EPD_Canvas Canvas(&M5.EPD);

Does.

[murraypaul]

Pull request: #20