[Touch] Webcam image is sometimes too dark

[Schwarzburg]

In the QML scanner version, the camera component is unstable. This bug should collect our information about this behaviour.

* On my desktop, the still images taken by the webcam with QML are sometimes too dark. It seems that the images are either all good or all bad between two starts of the app (not always however). Attatched are two screen recordings showing this behaviour.

* The viewfinder images (the video) are neither over nor underexposed.

* Output I get when runnging the touch-scanner app:

- every time:
"Focus mode selection is not supported"

- sometimes:
"virtual bool QSGVideoItemSurface::present(const QVideoFrame&) I'm getting bad frames here...
virtual bool QSGVideoItemSurface::present(const QVideoFrame&) I'm getting bad frames here... "

- once:
"CameraBin warning: "Internal data flow problem." "

* The webcam I use on the desktop is a built-in and has the following properties:
> v4l2-ctl --all
Driver Info (not using libv4l2):
        Driver name : uvcvideo
        Card type : USB2.0 HD UVC WebCam
        Bus info : usb-0000:00:1d.0-1.5
        Driver version: 3.8.13
        Capabilities : 0x84000001
                Video Capture
                Streaming
                Device Capabilities
        Device Caps : 0x04000001
                Video Capture
                Streaming
Priority: 2
Video input : 0 (Camera 1: ok)
Format Video Capture:
        Width/Height : 640/480
        Pixel Format : 'YUYV'
        Field : None
        Bytes per Line: 1280
        Size Image : 614400
        Colorspace : SRGB
Crop Capability Video Capture:
        Bounds : Left 0, Top 0, Width 640, Height 480
        Default : Left 0, Top 0, Width 640, Height 480
        Pixel Aspect: 1/1
Streaming Parameters Video Capture:
        Capabilities : timeperframe
        Frames per second: 30.000 (30/1)
        Read buffers : 0
        brightness (int) : min=-64 max=64 step=1 default=0 value=0
        contrast (int) : min=0 max=100 step=1 default=50 value=50
        saturation (int) : min=0 max=100 step=1 default=64 value=64
        hue (int) : min=-180 max=180 step=1 default=0 value=0
        white_balance_temperature_auto (bool) : default=1 value=1
        gamma (int) : min=100 max=500 step=1 default=300 value=300
        power_line_frequency (menu) : min=0 max=2 default=1 value=1
        white_balance_temperature (int) : min=2800 max=6500 step=10 default=4600 value=4600 flags=inactive
        sharpness (int) : min=0 max=100 step=1 default=50 value=50
        backlight_compensation (int) : min=0 max=1 step=1 default=0 value=0
        exposure_auto_priority (bool) : default=0 value=1

[Schwarzburg (stefan-schwarzburg)]

This is the second screen recording where the images are too dark.

[Schwarzburg (stefan-schwarzburg)]

Sometimes images are also half ok, half slightly too dark...