dim filter (#2323)

* dim filter

* optimization

* Remove test code

* move functions out of filter

* docstring

* move function to module scope

* docstring

* docstrings

tests/test_line_filter.py

@@ -0,0 +1,18 @@
+from rich.segment import Segment
+from rich.style import Style
+
+from textual.filter import DimFilter
+
+
+def test_dim_apply():
+    """Check dim filter changes color and resets dim attribute."""
+
+    dim_filter = DimFilter()
+
+    segments = [Segment("Hello, World!", Style.parse("dim #ffffff on #0000ff"))]
+
+    dimmed_segments = dim_filter.apply(segments)
+
+    expected = [Segment("Hello, World!", Style.parse("not dim #7f7fff on #0000ff"))]
+
+    assert dimmed_segments == expected