We have a number of places where we delete scene graph objects with
deleteLater() and change the scene graph. The timing of when the
deleteLater happens is very important as we need to complete
the updates to the scene graph first.

In this particular case, a QQuickShaderEffectSource was released,
and an update was scheduled but because animations were running we
were already beginning to render a new frame before the update was
handled, causing a crash.

The only safe place to run deferred deletes is after we have
performed a proper sync with the item scene.

Task-number: QTBUG-29485
Change-Id: I6e93d4e6276fe82d3f4c818781b188e03c46e510
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

The typical bottlenecks during rendering are usually compiling
shader programs, uploading textures and preparing glyphs,
so add profiling data around them.

Change-Id: I9c330a0f6b769836d303a035b2c0dce832842aec
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

This change starts using the superior implementation of the scene graph
render loop which has been worked on in the scenegraph-playground
project for a while. It uses a far more straightforward locking/sync
paradigm compared to the existing one and is less deadlock and error
prone. It also enables the scene graph thread to run on its own when
the GUI thread is blocked, enabling threaded animations.

This changes also introduces a naming change inside Qt Quick from
"Window Manager" -> "Render Loop" as that fits better to what the
code does.

Change-Id: I1c2170ee04fcbef79660bd7dae6cace647cdb276
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>