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_hardwarerenderer.cpp

@@ -0,0 +1,44 @@
+#include "qt_hardwarerenderer.hpp"
+
+void HardwareRenderer::resizeGL(int w, int h)
+{
+    glViewport(0, 0, w, h);
+}
+
+void HardwareRenderer::initializeGL()
+{
+    initializeOpenGLFunctions();
+}
+
+void HardwareRenderer::paintGL()
+{
+    QPainter painter(this);
+    painter.drawImage(QRect(0, 0, width(), height()), image, QRect(sx, sy, sw, sh));
+    // "release" image, reducing it's refcount, so renderstack::blit()
+    // won't have to reallocate
+    image = QImage();
+}
+
+void HardwareRenderer::setRenderType(RenderType type) {
+    QSurfaceFormat format;
+    switch (type) {
+    case RenderType::OpenGL:
+        setTextureFormat(GL_RGB);
+        format.setRenderableType(QSurfaceFormat::OpenGL);
+        break;
+    case RenderType::OpenGLES:
+        setTextureFormat(GL_RGBA);
+        format.setRenderableType(QSurfaceFormat::OpenGLES);
+        break;
+    }
+    setFormat(format);
+}
+
+void HardwareRenderer::onBlit(const QImage& img, int x, int y, int w, int h) {
+    image = img;
+    sx = x;
+    sy = y;
+    sw = w;
+    sh = h;
+    update();
+}