Add hint_transparent to use a transparent black placeholder texture

This can be used in shaders to avoid the need to supply a transparent
placeholder texture manually.

servers/rendering/shader_language.h

@@ -164,6 +164,7 @@ public:
 		TK_RENDER_MODE,
 		TK_HINT_DEFAULT_WHITE_TEXTURE,
 		TK_HINT_DEFAULT_BLACK_TEXTURE,
+		TK_HINT_DEFAULT_TRANSPARENT_TEXTURE,
 		TK_HINT_NORMAL_TEXTURE,
 		TK_HINT_ROUGHNESS_NORMAL_TEXTURE,
 		TK_HINT_ROUGHNESS_R,
@@ -664,6 +665,7 @@ public:
 				HINT_ROUGHNESS_GRAY,
 				HINT_DEFAULT_BLACK,
 				HINT_DEFAULT_WHITE,
+				HINT_DEFAULT_TRANSPARENT,
 				HINT_ANISOTROPY,
 				HINT_MAX
 			};