### Godot version

4.x

### System information

Any

### Issue description

Myself and @clayjohn looked into this before but were busy focusing on other things at the time so didn't investigate this further.

In the scene shader of the compatibility renderer, the forward and mobile implementation we have a variation of this check:
```
#ifdef USE_OPAQUE_PREPASS
	if (alpha < scene_data.opaque_prepass_threshold) {
		discard;
	}
#endif // USE_OPAQUE_PREPASS
```

As best as I can tell USE_OPAQUE_PREPASS is set by default for our StandardMaterial so this code is always included however for normal fully opaque materials we wouldn't want this code to be included especially when we're optimising for depth prepass or shadow rendering and want the fragment code completely filtered out. The scenario we were investigating had this introducing a needless texture read because it couldn't filter out this check. While it was inconclusive how much impact this had, theory states that including this when not needed has a detrimental impact.

I'm not entirely sure what the check is supposed to accomplish but it seems to only be applicable when rendering transparent objects during depth pre-pass or shadow rendering.

### Steps to reproduce

Create any scene that includes a `StandardMaterial` and uses an albedo texture. Examine the shader code sent to the depth pre-pass and shadow passes.

### Minimal reproduction project

n/a
actions.usage_defines["CLEARCOAT"] = "#define LIGHT_CLEARCOAT_USED\n";
		actions.usage_defines["CLEARCOAT_ROUGHNESS"] = "@CLEARCOAT";
		actions.usage_defines["ANISOTROPY"] = "#define LIGHT_ANISOTROPY_USED\n";
		actions.usage_defines["ANISOTROPY_FLOW"] = "@ANISOTROPY";
		actions.usage_defines["AO"] = "#define AO_USED\n";
		actions.usage_defines["AO_LIGHT_AFFECT"] = "#define AO_USED\n";
		actions.usage_defines["UV"] = "#define UV_USED\n";
		actions.usage_defines["UV2"] = "#define UV2_USED\n";
		actions.usage_defines["BONE_INDICES"] = "#define BONES_USED\n";
		actions.usage_defines["BONE_WEIGHTS"] = "#define WEIGHTS_USED\n";
		actions.usage_defines["CUSTOM0"] = "#define CUSTOM0_USED\n";
		actions.usage_defines["CUSTOM1"] = "#define CUSTOM1_USED\n";
		actions.usage_defines["CUSTOM2"] = "#define CUSTOM2_USED\n";
		actions.usage_defines["CUSTOM3"] = "#define CUSTOM3_USED\n";
		actions.usage_defines["NORMAL_MAP"] = "#define NORMAL_MAP_USED\n";
		actions.usage_defines["NORMAL_MAP_DEPTH"] = "@NORMAL_MAP";
		actions.usage_defines["COLOR"] = "#define COLOR_USED\n";
		actions.usage_defines["INSTANCE_CUSTOM"] = "#define ENABLE_INSTANCE_CUSTOM\n";
		actions.usage_defines["POSITION"] = "#define OVERRIDE_POSITION\n";
		actions.usage_defines["ALPHA_SCISSOR_THRESHOLD"] = "#define ALPHA_SCISSOR_USED\n";
		actions.usage_defines["ALPHA_HASH_SCALE"] = "#define ALPHA_HASH_USED\n";
		actions.usage_defines["ALPHA_ANTIALIASING_EDGE"] = "#define ALPHA_ANTIALIASING_EDGE_USED\n";
		actions.usage_defines["ALPHA_TEXTURE_COORDINATE"] = "@ALPHA_ANTIALIASING_EDGE";
		actions.usage_defines["SSS_STRENGTH"] = "#define ENABLE_SSS\n";
		actions.usage_defines["SSS_TRANSMITTANCE_DEPTH"] = "#define ENABLE_TRANSMITTANCE\n";
		actions.usage_defines["BACKLIGHT"] = "#define LIGHT_BACKLIGHT_USED\n";
		actions.usage_defines["SCREEN_UV"] = "#define SCREEN_UV_USED\n";
		actions.usage_defines["DIFFUSE_LIGHT"] = "#define USE_LIGHT_SHADER_CODE\n";
		actions.usage_defines["SPECULAR_LIGHT"] = "#define USE_LIGHT_SHADER_CODE\n";
		actions.usage_defines["FOG"] = "#define CUSTOM_FOG_USED\n";