render-to-texture widgets may stop animating on eglfs

Run the qopenglwidget example on eglfs. Move the cursor around and use the other widgets, for example the scroll bars on the scroll area. Sometimes the QOpenGLWidget will stop animating and will receive no further updates until it is triggered by some other widget.

The issue is specific to eglfs. This indicates that it originates somewhere in the platformsupport/platformcompositor code and the QPlatformTextureList's locking mechanism since this is the only platform that currently features a built-in compositor and thus has to deal with deferring backingstore flushes when necessary.