The lack of icon gizmo made it no longer possible to select Camera3D
nodes in the 3D viewport by clicking on them.
A new Camera3D icon gizmo was designed to be direction-neutral.
This avoids the issue that the previous icon gizmo had, since it was
always facing right.
This also adds icons for CameraAttributesPractical and CameraAttributesPhysical.
This allows light sources to be specified in physical light units in addition to the regular energy multiplier. In order to avoid loss of precision at high values, brightness values are premultiplied by an exposure normalization value.
In support of Physical Light Units this PR also renames CameraEffects to CameraAttributes.
