Cleanup: Silent Warnings

Silent unused variable warnings introduced by recent changes in the
overlay engine.

Fix T72154: Background Camera Images

The Camera Background Images uses a alpha under blending. For alpha
under blending to work correctly the framebuffer containing the result
of the render engine needs to be active
(`DefaultFramebufferList.default_fb`) and the source blend color needs
to be premultiplied with alpha.

Due to recent refactoring this wasn't the case and it seemed that the
background image was drawn in front of the scene. This patch sets the
correct state so it seems to be drawn behind the scene.

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D6365

Fix T72175: DrawManager Crash Large Objects

Introduced by 9c337fcfe2a4 mistaken that `MeshExtract.use_threading` set
to false means that no threading is used at all. This is not the case it
will still perform threading for large objects, it will only run the
different subtasks in serial.

Fixed by scheduling the `lines_loose` in the task_pool after the rest
have been executed. This is cleaner than the previous implementation as
it sticks more to the actual design.

Fix T72157: Overlay Hide Bone Relationship Lines

Bone relationship lines needs to be hidden in:

* object mode
* or when relationship lines are turned off

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D6356

Fix T72130: Wireframe Visible After Disabling

The lines index buffer can contain all edges (edit mode) or only loose
edges (object mode). When switching between these modes the wrong
content of the index buffer can be used.

This patch will clear the lines index buffer when a `loose_edges` is requested. Making sure it is always up to date.

Note that this is supporting an exising hack where the IBO is truncated
during the creation. We should find a different way how to solve these
kind of issues.

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D6349

Fix T72156: Render Viewport Animation Meta Data

When using Render Viewport Animation the meta data
was only stamped when a float buffer was used. In recent changes it was
possible to use a char buffer for the result.

This commit will also support stamping of metadata using a char buffer.

DrawManager: Bounding Box Drawing

Fix for T72136: Bounding boxes were still drawn even with disabled
overlays. Also the outline displayed the original mesh

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D6350

EEVEE: Cleanup

Remove redundant defines.

EEVEE: RenderLayers

Small change do not draw an unsupported renderpass in the viewport

EEVEE: Viewport Renderpasses

This patch will allow the user to select the EEVEE renderpass to be
shown in the viewport by default the combined pass will be shown.

Limitations:

* Viewport rendering stores the result in a `RenderResult`. RenderResult
is not aware of the type of data it holds. In many places where RenderResult
is used it is assumed that it stores a combined pass and the display+view
transform are applied.

I will propose to fix this in a future patch. But that is still being
designed and discussed.

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D6319