added software renderer. split common functions prev.

in gleswidget to renderstack (a QStackWidget), which in
turn calls an actual renderer. added ability to target GLES
directly, but this is maybe uneeded.

src/qt/qt_softwarerenderer.hpp

@@ -0,0 +1,22 @@
+#ifndef SOFTWARERENDERER_HPP
+#define SOFTWARERENDERER_HPP
+
+#include <QWidget>
+
+class SoftwareRenderer : public QWidget
+{
+    Q_OBJECT
+public:
+    explicit SoftwareRenderer(QWidget *parent = nullptr);
+
+    void paintEvent(QPaintEvent *event) override;
+
+public slots:
+    void onBlit(const QImage& img, int, int, int, int);
+
+private:
+    QImage image;
+    int sx, sy, sw, sh;
+};
+
+#endif // SOFTWARERENDERER_HPP