RenderingServer - Add function to check if global shader parameter exists

[OhiraKyou]

I could use a function to check if RenderingServer is already aware of a global shader parameter, such as when it was added to the project settings interface or via script.

Trying to get a property by name and checking for null doesn't work for objects, like the sampler2d type—those return null if they're simply not set, even when they are registered in the project settings. Example workaround code that doesn't work as a result:

func _set_global(global_name: String, global_type: RenderingServer.GlobalShaderParameterType, value):
	var existing = RenderingServer.global_shader_parameter_get(global_name)
	
	# Only add globals if they have already been created in the project settings
	# This fails on objects, like textures, which may be null in settings
	if existing == null:
		RenderingServer.global_shader_parameter_add(global_name, global_type, value)
	else:
		RenderingServer.global_shader_parameter_set(global_name, value)