Fix using AugmentShaderDirty flag

The current implementation doesn't use the flag which means that the
whole augmented shader processing is redone every time something changes
in the render pass, not just when the shader changes.

There is already AugmentedShaderDirty flag so start using it in the
updateSpatialNode.

Pick-to: 6.12 6.11
Change-Id: Ib24f6603b2fe2173a8187152a119237a622eb280
Reviewed-by: Christian Strømme <christian.stromme@qt.io>