canvas.paintEvent() bug fix

https://github.com/PaddlePaddle/PaddleOCR/issues/7681 
https://github.com/PaddlePaddle/PaddleOCR/issues/7702 
self.prevPoint存储使用浮点型，p.drawLine只接受整型，触发上述异常。

PPOCRLabel/libs/canvas.py

@@ -611,8 +611,8 @@ class Canvas(QWidget):
 
         if self.drawing() and not self.prevPoint.isNull() and not self.outOfPixmap(self.prevPoint):
             p.setPen(QColor(0, 0, 0))
-            p.drawLine(self.prevPoint.x(), 0, self.prevPoint.x(), self.pixmap.height())
-            p.drawLine(0, self.prevPoint.y(), self.pixmap.width(), self.prevPoint.y())
+            p.drawLine(int(self.prevPoint.x()), 0, int(self.prevPoint.x()), self.pixmap.height())
+            p.drawLine(0, int(self.prevPoint.y()), self.pixmap.width(), int(self.prevPoint.y()))
 
         self.setAutoFillBackground(True)
         if self.verified:
@@ -909,4 +909,4 @@ class Canvas(QWidget):
     def updateShapeIndex(self):
         for i in range(len(self.shapes)):
             self.shapes[i].idx = i
-        self.update()
+        self.update()
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