Add hint_transparent to use a transparent black placeholder texture

servers/visual/shader_language.h

@@ -157,6 +157,7 @@ public:
 		TK_RENDER_MODE,
 		TK_HINT_WHITE_TEXTURE,
 		TK_HINT_BLACK_TEXTURE,
+		TK_HINT_TRANSPARENT_TEXTURE,
 		TK_HINT_NORMAL_TEXTURE,
 		TK_HINT_ANISO_TEXTURE,
 		TK_HINT_ALBEDO_TEXTURE,
@@ -643,6 +644,7 @@ public:
 				HINT_NORMAL,
 				HINT_BLACK,
 				HINT_WHITE,
+				HINT_TRANSPARENT,
 				HINT_ANISO,
 				HINT_MAX
 			};