Get rid of depth buffer workaround in the render pipeline code (#15407)

I originally wanted to get of the legacy IVideoDriver::setRenderTarget altogether,
but that ended up being too much work.
The remaining usage is in "dynamicshadowsrender.cpp".

Here's a comment I wrote about the workaround:

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Use legacy call when there's single texture without depth texture
This means Irrlicht creates a depth texture for us and binds it to the FBO

This is currently necessary for a working depth buffer in the following cases:

- post-processing disabled, undersampling enabled
  (addUpscaling specifies no depth texture)

- post-processing disabled, 3d_mode = sidebyside / topbottom / crossview
  (populateSideBySidePipeline specifies no depth texture)

- post-processing disabled, 3d_mode = interlaced
  (probably, can't test since it's broken)
  (populateInterlacedPipeline specifies no depth texture)

With post-processing disabled, the world is rendered to the TextureBufferOutput
created in the functions listed above, so a depth buffer is needed
(-> this workaround is needed).
With post-processing enabled, only a fullscreen rectangle is rendered to
this TextureBufferOutput, so a depth buffer isn't actually needed.
But: These pipeline steps shouldn't rely on what ends up being rendered to
the TextureBufferOutput they provide, since that may change.

This workaround was added in 1e96403954 /
https://irc.minetest.net/minetest-dev/2022-10-04#i_6021940

This workaround should be replaced by explicitly configuring depth
textures where needed.

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builtin/settingtypes.txt

@@ -263,6 +263,8 @@ viewing_range (Viewing range) int 190 20 4000
 #    to the game world only, keeping the GUI intact.
 #    It should give a significant performance boost at the cost of less detailed image.
 #    Higher values result in a less detailed image.
+#    Note: Undersampling is currently not supported if the "3d_mode" setting is set
+#    to a non-default value.
 undersampling (Undersampling) int 1 1 8
 
 [**3D]