qml6glsrc: Reduce capture delay

qml6glsrc: Reduce capture delay

In qml6glsrc, we capture the application by copying the back buffer into
our own FBO. The afterRendering() signal is too soon as from the apitrace, the
application has been rendered into a QT internal buffer, to be used as a cache
for refresh.

Use afterFrameEnd() signal instead. This works with no delay on GLES. With GL
it seems to reduce from 2 to 1 frame delay (this may be platform specific). A
different recording technique would need to be used to completely remove this
delay.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7562>