Document how to use env QT_GSTREAMER_OVERRIDE_VIDEO_CONVERSION_ELEMENT

The environment variable QT_GSTREAMER_OVERRIDE_VIDEO_CONVERSION_ELEMENT can be used to configure the GStreamer pipeline to use GLMemory for video rendering. On embedded devices that don't support rendering with DMA_BUF memory, this is needed to avoid high CPU load.

For example on NXP imx8m plus, GLMemory rendering with QMediaPlayer can be obtained by setting

QT_GSTREAMER_OVERRIDE_VIDEO_CONVERSION_ELEMENT=glupload ! glcolorconvert

, which adds the gstreamer elements glupload and glcolorconvert into the pipeline before Qt's qgstvideorenderersink.