Text overdraw maintains ANSI characters from behind

[mattprecious]

Failing test:

@Test
fun overdraw() = runTest {
  runMosaicTest(RenderingSnapshots(rendering)) {
    setContent {
      Box {
        Text(
          buildAnnotatedString {
            withStyle(SpanStyle(textStyle = Underline)) {
              append("Hello")
            }
          }
        )

        Text(
          modifier = Modifier.background(Color(0xffffff)),
          value = "World",
        )
      }
    }

    assertThat(awaitSnapshot()).isEqualTo(
      """
      |${CSI}48;2;255;255;255mWorld${CSI}0m
      |
      """.trimMargin().wrapWithAnsiSynchronizedUpdate().replaceLineEndingsWithCRLF(),
    )
  }
}

It's debatable whether this is a bug or not, but explicitly adding a background color to the second text makes it a stronger argument that the underline should be stripped from the merged output. Otherwise, it's not going to be possible to build UIs with z-index.

Removing the background color and just doing Text("World") could also arguably not have an underline in the merged output, but the counter argument is that technically you would still see the underline because it's below the characters. What about Strikethrough and text color though?

[JakeWharton]

This is surprisingly hard. I've been previously using a space to mean empty, but I might need to differentiate between actually-empty and is-a-space in order to know whether we should clear styles on write.