Qt: Fix changing resolution of software renderer

CHANGES

@@ -5,6 +5,7 @@ Bugfixes:
  - GBA BIOS: Fix invalid CpuSet not setting BIOS prefetch
  - GB MBC: Fix SRAM dangling pointer with RTC games
  - Windows: Fix Unicode directory handling
+ - Qt: Fix changing resolution of software renderer
 
 0.5.1: (2016-10-05)
 Bugfixes:

src/platform/qt/DisplayQt.cpp

@@ -23,6 +23,7 @@ DisplayQt::DisplayQt(QWidget* parent)
 
 void DisplayQt::startDrawing(mCoreThread* context) {
 	context->core->desiredVideoDimensions(context->core, &m_width, &m_height);
+	m_backing = std::move(QImage());
 	m_isDrawing = true;
 }