Speed up painting of qr codes.

Probably speeds it up by about a factor of two.
Unfortunately it needs to be another 5x faster
for sluggishness to disappear in the GUI when
typing a description in the receive tab.

Note the old code was off-by-one.

gui/qt/qrcodewidget.py

@@ -72,16 +72,13 @@ class QRCodeWidget(QWidget):
         qp.setBrush(white)
         qp.setPen(white)
         qp.drawRect(left-margin, top-margin, size+(margin*2), size+(margin*2))
+        qp.setBrush(black)
+        qp.setPen(black)
 
         for r in range(k):
             for c in range(k):
                 if matrix[r][c]:
-                    qp.setBrush(black)
-                    qp.setPen(black)
-                else:
-                    qp.setBrush(white)
-                    qp.setPen(white)
-                qp.drawRect(left+c*boxsize, top+r*boxsize, boxsize, boxsize)
+                    qp.drawRect(left+c*boxsize, top+r*boxsize, boxsize - 1, boxsize - 1)
         qp.end()