Draw frustum splices on top of direction shadow atlas for debug purposes

[BastiaanOlij]

To help debug issues with directional shadows this PR introduces a mechanism of drawing the view frustum splices into the directional atlas when displayed in the interface:

This will help with setting up better splits and to see if we're effectively culling out objects that do not need to be drawn into the shadow map.

[BastiaanOlij]

Added some wireframe lines to make it easier to recognise the frustums.

Note that the frustums drawn are based on their actual application, so we break the main frustum up like so:

If overlap is enabled the drawn frustums include the overlap.

[Calinou]

If overlap is enabled the drawn frustums include the overlap.

This should be mentioned in the class reference 🙂

[BastiaanOlij]

This should be mentioned in the class reference 🙂

@Calinou added a description, let me know if this is clear enough

[BastiaanOlij]

One more change I thought was worth it, the shadowmaps are squashed vertically because we're rendering to 1/4th of the screen. The shadowmaps however are a square. So I changed the logic to keep the aspect ratio:

[BastiaanOlij]

And one more little enhancement. The last cascade now shows all slices with only the final slice having a wireframe:

This more clearly communicates the relationship between the slices and the total coverage.

[clayjohn]

Just some nitpicky formatting comments, otherwise this looks good to go

[BastiaanOlij]

@clayjohn @Calinou if you're happy with the changes, any chance to approve this PR so it can be merged once we're working on 4.2?

[Calinou]

Tested locally (rebased against 0f76ff2), it works.

Blend Splits disabled

Blend Splits enabled

[YuriSizov]

Thanks!