Fix flaky FP32x4 test

The composition implementation may act unexpectedly on Inf or NaN input
values. This change avoid those values, without changing the current
implementation.

Pick-to: 6.4
Fixes: QTBUG-101236
Change-Id: I8e4ee67f53093b7f81e014b28d8a028ba2ddcc47
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

tests/auto/gui/painting/qpainter/tst_qpainter.cpp

@@ -5506,6 +5506,7 @@ void tst_QPainter::hdrColors()
     QCOMPARE(img.pixelColor(4, 4), color);
 
     QImage img2(10, 10, QImage::Format_RGBX32FPx4);
+    img2.fill(Qt::black); // fill to avoid random FP values like Inf which can break SourceOver composition
     {
         QPainter p(&img2);
         p.drawImage(0, 0, img);