gl: bind to old fbo when glsl is applied

we can revert qml QuickFBORenderer commit about fbo rebinding

src/QtAV/GLSLFilter.h

@@ -65,6 +65,13 @@ class Q_AV_EXPORT GLSLFilter : public VideoFilter
     void outputSizeChanged(const QSize& size);
 protected:
     GLSLFilter(GLSLFilterPrivate& d, QObject *parent = 0);
+    /*!
+     * \brief process
+     * Draw video frame into fbo and apply the user shader from opengl()->userShader();
+     * \param frame input frame can be a frame holding host memory data, or any other GPU frame can interop with OpenGL texture (including frames from HW decoders in QtAV).
+     * Output frame holds an RGB texture, which can be processed in the next GPU filter, or rendered by OpenGL renderers.
+     * When process() is done, FBO before before process() is bounded.
+     */
     void process(Statistics* statistics, VideoFrame* frame = 0) Q_DECL_OVERRIDE;
 };
 } //namespace QtAV

src/filter/GLSLFilter.cpp

@@ -91,6 +91,8 @@ void GLSLFilter::process(Statistics *statistics, VideoFrame *frame)
     DPTR_D(GLSLFilter);
     if (!frame || !*frame)
         return;
+    GLint currentFbo = 0;
+    DYGL(glGetIntegerv(GL_FRAMEBUFFER_BINDING, &currentFbo));
     // now use the frame size
     if (d.fbo) {
         if (d.size.isEmpty()) {
@@ -118,8 +120,7 @@ void GLSLFilter::process(Statistics *statistics, VideoFrame *frame)
     QMatrix4x4 mat; // flip vertical
     mat.scale(1, -1);
     d.glv.render(QRectF(), QRectF(), mat);
-    QOpenGLFramebufferObject::bindDefault();
-
+    DYGL(glBindFramebuffer(GL_FRAMEBUFFER, (GLuint)currentFbo));
     VideoFormat fmt(VideoFormat::Format_RGB32);
     VideoFrame f(d.fbo->width(), d.fbo->height(), fmt); //
     f.setBytesPerLine(d.fbo->width()*fmt.bytesPerPixel(), 0);