qmlgl6src: Fix crash when use-default-fbo is not set

At its default, use-default-fbo property on qml6glsrc leads to a crash. This is fixed by not unmapping the GL video frame twice.

The feature is still broken after fixing the crash. QT reports this.

Custom render target is set but no command buffer has been provided.

I did not yet figure-out yet what is needed, but I believe that mode just never worked, and its slightly annoying considering this is the property default value. From testing the QT5 version, I believe that this should make the QT app render into our target only (no on-screen rendering). This can be more efficient when streaming only.

cc @ystreet