Fix use of discard in shaders

[clayjohn]

discard was being included in all shaders set to depth pass opaque, which is the majority of shaders. Instead it should only be used with alpha prepass materials as it disables some GPU optimizations.

This PR also cleans up the shader defines a bit to ensure that the OPAQUE_PREPASS discard is only included when in the depth pass, and when not using an alpha hash.

Addresses the first concern in godotengine/godot-proposals#7366

Testing with the Bistro reference scene I get anywhere from a 2% to a 5% decrease in frame times (when GI and shadows are disabled, meaning the majority of the cost is depth pass and opaque pass).

Production edit: Fixes #77600

[reduz]

Not sure if I understand the change on the shader, but remember it also needs to run on the depth passes.

[clayjohn]

Not sure if I understand the change on the shader, but remember it also needs to run on the depth passes.

Thanks. This logic ensures that it only runs when in the depth pass and we are using a depth_draw_alpha_prepass material. Regular transparent, transparent hash, and opaque objects never need this

[BastiaanOlij]

Indeed, this is especially important when rendering opaque objects, just including the discard check if if totally defunct means we get the overhead of the fragment shader running.

Should fix: #77600

[YuriSizov]

Thanks!