Ensure members are initialized in compatibility renderer SceneShaderData

[RoyBerube]

Fixes issue #84418

Boolean variables that were uninitialized in the struct SceneShaderData could cause an error message if never set to a value.

This fixes the problem by initializing all boolean variables of SceneShaderData to false in the constructor. This avoids the errors caused by using undefined values.

[clayjohn]

These variable need to be initialized every time that set_code() is called. It isn't enough to initialize them when the class is created for the first time since set_code() can be called multiple times.

You can see that most properties are correctly initialized at the top of set_code():

godot/drivers/gles3/storage/material_storage.cpp

Lines 2860 to 2871 in 3e7f638

	unshaded = false;
	uses_vertex = false;
	uses_position = false;
	uses_sss = false;
	uses_transmittance = false;
	uses_screen_texture = false;
	uses_depth_texture = false;
	uses_normal_texture = false;
	uses_time = false;
	writes_modelview_or_projection = false;
	uses_world_coordinates = false;
	uses_particle_trails = false;

We just need to initialize the rest in the same place

[akien-mga]

Superseded by #84752, which takes the approach recommended by @clayjohn, and applies the same treatment to all other ShaderData classes.