[nemo-qtmultimedia-plugins] videotexturebackend: call attached QML video filters. Contributes to MER#1897
Fix Qt Multimedia declarative video backend plugin not applying attached video filters:
- Add overrides for QDeclarativeVideoBackend::appendFilter(), clearFilters() to keep track of attached filters locally
- Add TextureVideoBuffer class
- Use TextureVideoBuffer to implement rendering video frame to offscreen framebuffer object
- Use glReadPixels() to get current frame's pixel data
- Call video filters chain from GStreamerVideoTexture::updateTexture()
With this fix video filters are read-only, and can be used only to analyze incoming video stream in real time. Ideally filters should be able to modify every frame's pixels, but rendering modified video frame back to texture is not implemented now.