Fix compatibility renderer depth_prepass_alpha

[Khasehemwy]

In compatibility mode, when using render_mode depth_prepass_alpha; in Shader, the depth pre pass is not work.

I tested minimal project in Forward+ mode, Nsight showed that a depth pre pass was performed. But I didn't see it in compatibility mode. So I modified the OpenGL part according to the code like render_forward_clustered.

Minimum test project:
test_depth_prepass_alpha_compatibility.zip

[clayjohn]

This looks great. Thank you.

For context this is the same behaviour as the Forward+ Renderer, the Mobile renderer is different because it doesn't do a depth prepass.

This adds alpha prepass objects to both render lists, but the render_list_template function ensures that alpha-prepass objects are not drawn in the forward pass

godot/drivers/gles3/rasterizer_scene_gles3.cpp

Lines 2931 to 2933 in a0943ac

	if (p_pass_mode == PASS_MODE_COLOR && !(surf->flags & GeometryInstanceSurface::FLAG_PASS_OPAQUE)) {
	continue; // Objects with "Depth-prepass" transparency are included in both render lists, but should only be rendered in the transparent pass
	}

[akien-mga]

Thanks!