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:

----------------------------------------

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.

----------------------------------------

src/client/render/interlaced.cpp

@@ -35,6 +35,13 @@ void InitInterlacedMaskStep::run(PipelineContext &context)
 
 void populateInterlacedPipeline(RenderPipeline *pipeline, Client *client)
 {
+	// FIXME: "3d_mode = interlaced" is currently broken. Two options:
+	// 1. Remove it
+	// 2. Fix it
+	// If you fix it, make sure to test it with "enable_post_processing = false".
+	// You'll probably have to add a depth texture to make that combination work.
+	// Also, this code should probably use selectColorFormat/selectDepthFormat.
+
 	static const u8 TEXTURE_LEFT = 0;
 	static const u8 TEXTURE_RIGHT = 1;
 	static const u8 TEXTURE_MASK = 2;