Qt: Make QrCodeWidget paint things properly on high-dpi displays
Qt: Make QrCodeWidget paint things properly on high-dpi displays

Taken from Electrum: 93724ba33a6d6601efd0fee571863fa2d8a3a460

Basically the key missing piece was specifying Qt.MiterJoin.  This makes
the QR codes generated by the app look ok on high-dpi whereas before you
could see little white dots where the boxes joined.

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