tdf#120777 KDE5: Update initialization of QImages

Make sure that created empty QImages are filled with transparent pixels.

Copying data from previous QImage on widget resize removes blanking
on window resize when qt5 vcl plugin is used.

Change-Id: If072a4b8b334bc87dbe4aaea9aa8774bb5e202ee
Reviewed-on: https://gerrit.libreoffice.org/63029Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
Tested-by: Katarina Behrens <Katarina.Behrens@cib.de>

vcl/qt5/Qt5Bitmap.cxx

@@ -63,6 +63,7 @@ bool Qt5Bitmap::Create(const Size& rSize, sal_uInt16 nBitCount, const BitmapPale
     else
     {
         m_pImage.reset(new QImage(toQSize(rSize), getBitFormat(nBitCount)));
+        m_pImage->fill(Qt::transparent);
         m_pBuffer.reset();
     }
     m_aPalette = rPal;
@@ -129,7 +130,7 @@ bool Qt5Bitmap::Create(const SalBitmap& rSalBmp, sal_uInt16 nNewBitCount)
         // convert 4bit indexed palette to 32bit ARGB
         m_pImage.reset(new QImage(pBitmap->m_aSize.Width(), pBitmap->m_aSize.Height(),
                                   getBitFormat(nNewBitCount)));
-        m_pImage->fill(0);
+        m_pImage->fill(Qt::transparent);
 
         // prepare a whole palette
         const BitmapPalette& rPal = pBitmap->m_aPalette;

vcl/qt5/Qt5Frame.cxx

@@ -211,6 +211,7 @@ SalGraphics* Qt5Frame::AcquireGraphics()
         {
             m_pQt5Graphics.reset(new Qt5Graphics(this));
             m_pQImage.reset(new QImage(m_pQWidget->size(), Qt5_DefaultFormat32));
+            m_pQImage->fill(Qt::transparent);
             m_pQt5Graphics->ChangeQImage(m_pQImage.get());
         }
         return m_pQt5Graphics.get();

vcl/qt5/Qt5VirtualDevice.cxx

@@ -76,6 +76,7 @@ bool Qt5VirtualDevice::SetSizeUsingBuffer(long nNewDX, long nNewDY, sal_uInt8* p
             m_pImage.reset(new QImage(nNewDX, nNewDY, Qt5_DefaultFormat32));
     }
 
+    m_pImage->fill(Qt::transparent);
     m_pImage->setDevicePixelRatio(m_fScale);
 
     // update device in existing graphics

vcl/qt5/Qt5Widget.cxx

@@ -86,7 +86,17 @@ void Qt5Widget::resizeEvent(QResizeEvent* pEvent)
     }
     else
     {
-        QImage* pImage = new QImage(size(), Qt5_DefaultFormat32);
+        QImage* pImage = nullptr;
+
+        if (m_pFrame->m_pQImage)
+            pImage = new QImage(
+                m_pFrame->m_pQImage->copy(0, 0, pEvent->size().width(), pEvent->size().height()));
+        else
+        {
+            pImage = new QImage(size(), Qt5_DefaultFormat32);
+            pImage->fill(Qt::transparent);
+        }
+
         m_pFrame->m_pQt5Graphics->ChangeQImage(pImage);
         m_pFrame->m_pQImage.reset(pImage);
     }