Make LA transform fill opaque, consistent with RGBA

test_fill[LA] asserted (76, 0): a transparent fill for an opaque "red"
fillcolor. That was a side effect of getcolor("red", "La") returning a
bare graylevel int, so the transform's internal La fill defaulted alpha
to 0. With getcolor now returning (76, 255), the LA fill matches both the
RGBA case and Image.new("LA", ..., "red").

Author: chuenchen309

Tests/test_image_transform.py

@@ -94,7 +94,7 @@ def test_quad(self) -> None:
         (
             ("RGB", (255, 0, 0)),
             ("RGBA", (255, 0, 0, 255)),
-            ("LA", (76, 0)),
+            ("LA", (76, 255)),
         ),
     )
     def test_fill(self, mode: str, expected_pixel: tuple[int, ...]) -> None: