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.cpp

@@ -0,0 +1,22 @@
+#include "qt_softwarerenderer.hpp"
+
+#include <QPainter>
+
+SoftwareRenderer::SoftwareRenderer(QWidget *parent) : QWidget(parent) {}
+
+void SoftwareRenderer::paintEvent(QPaintEvent *event) {
+    (void) event;
+
+    QPainter painter(this);
+    painter.drawImage(QRect(0, 0, width(), height()), image, QRect(sx, sy, sw, sh));
+    image = QImage();
+}
+
+void SoftwareRenderer::onBlit(const QImage& img, int x, int y, int w, int h) {
+    image = img;
+    sx = x;
+    sy = y;
+    sw = w;
+    sh = h;
+    update();
+}