Fix qssgrenderuserpass_p.h using wrong flag variable

The class uses flags from the base class to dirty and clear the
ShaderDirty flag, which conflicts with the base class's
HasGraphicsResources. It should be using the m_dirtyFlags.

The class has been using the HasGraphicsResources, so calling
clearDirty(DirtyFlag::ShaderDirty) has been clearing that flag.
The scene manager then deleted such a pass directly instead of
queueing it for cleanup on the render thread, so
QSSGBufferManager::releaseUserRenderPass() was never called and
the pass was not unscheduled from the user render pass managers.

Pick-to: 6.12
Change-Id: Icbca0a209d007da9ea1f36117e3f1208e61e0d72
Reviewed-by: Christian Strømme <christian.stromme@qt.io>