Add adjustments and color correction to Compatibility renderer

[clayjohn]

Fixes: #66457

This is the last of the major features missing from the Compatibility renderer. The remaining features are long term projects (e.g. VoxelGI) or things that won't be supported (e.g. SDFGI).

Using adjustments forces using the post processing pass (which is bad for performance on mobile devices). Originally we wanted to implement this in the scene shader and only use the post processing pass with glow. However, the BCS effects are not suitable for multipass lighting. Brightness, saturation and color correction were broken, but acceptable, but contrast completely breaks to a degree that isn't acceptable. I.e. when using contrast any object with multipass lighting has the reverse of the contrast used.

[clayjohn]

This fixes a bug that became more obvious as a result of enabling post effects.

This code matches the behaviour of the RD renderers:

godot/servers/rendering/renderer_rd/shaders/forward_clustered/scene_forward_clustered.glsl

Lines 1066 to 1068 in 11d3768

	#if (defined(ALPHA_SCISSOR_USED) || defined(ALPHA_HASH_USED)) && !defined(ALPHA_ANTIALIASING_EDGE_USED)
	alpha = 1.0;
	#endif

[BastiaanOlij]

LGTM

[akien-mga]

Thanks! Awesome to have the Compatibility renderer basically feature complete for 4.3 beta!

[Bimbam360]

Just span up 4.3dev6 and adjustments aren't applying on a newly created scene with a new spatial material and albedo applied:

Compatibility Behaviour

At least there's no more errors, but also seemingly does nothing.

For reference:
Forward Expected Behaviour

[clayjohn]

@Bimbam360 sorry about that. A bug slipped through and the regular adjustments get disabled unless color correction texture is used. It's fixed already in master and will be fixed in beta 1

[naturally-intelligent]

Thanks for adding this! After glow, this was the last feature missing that brings our project back to full v3 compatibility.