Set Default Value for Slate Arguments

SLATE_BEGIN_ARGS is a constructor, so you can set default values there.

SLATE_BEGIN_ARGS(SSH_TextureOutput)
	: _NullTextureColour(FLinearColor(0.1f, 0.f, 0.f))
	, _Title("OUTPUT TEXTURE")
{}
	SLATE_ARGUMENT(FLinearColor, NullTextureColour)
	SLATE_ARGUMENT(FString, Title)
SLATE_END_ARGS()