ShaderGathererJob should depend on FilterCompatibleTechniquesJob

The ShaderGathererJob will use Technique::isCompatibleWithRenderer() while looking for dirty shaders. This is set in the FilterCompatibleTechniquesJob, but it might run after (or, even worse, at the same time as) ShaderGathererJob.

I'm including a fix for this (among other things) in this change, but we might want to merge a quick fix in the meanwhile:

https://codereview.qt-project.org/#/c/217827/