Add highlighter opacity.

feature/media-send/src/main/java/org/signal/mediasend/edit/ImageController.kt

@@ -191,7 +191,7 @@ class ImageController @RememberInComposition constructor(
   }
 
   fun setDrawColor(color: Int) {
-    imageEditorState.drawColor = color
+    imageEditorState.drawColor = brushTool?.applyAlpha(color) ?: color
   }
 
   fun setBrushWidthFraction(fraction: Float) {
@@ -205,7 +205,7 @@ class ImageController @RememberInComposition constructor(
     imageEditorState.isBlur = mode == Mode.BLUR
     imageEditorState.drawCap = if (mode == Mode.HIGHLIGHT) Paint.Cap.SQUARE else Paint.Cap.ROUND
     imageEditorState.drawThickness = brushThickness
-    imageEditorState.drawColor = drawColorBarState.color
+    setDrawColor(drawColorBarState.color)
   }
 
   fun enterCropMode() {

feature/media-send/src/main/java/org/signal/mediasend/edit/MediaEditScreen.kt

@@ -193,7 +193,7 @@ fun MediaEditScreen(
         visible = isAdjustingBrushWidth,
         thickness = imageController.brushThickness,
         viewMatrix = imageController.imageEditorState.viewMatrix,
-        color = Color(imageController.imageEditorState.drawColor),
+        color = Color(imageController.drawColorBarState.color),
         isBlur = imageController.isUserBlurring,
         modifier = Modifier.fillMaxSize()
       )

feature/media-send/src/main/java/org/signal/mediasend/edit/image/BrushWidths.kt

@@ -11,20 +11,27 @@ import androidx.compose.runtime.annotation.RememberInComposition
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.setValue
+import androidx.core.graphics.ColorUtils
 import kotlinx.parcelize.Parcelize
 
+private const val OPAQUE_ALPHA = 0xFF
+private const val HIGHLIGHTER_ALPHA = 0x60
+
 /**
  * The image editor tools which support a user-adjustable stroke width.
  *
  * Thicknesses are expressed as a fraction of the editor's coordinate space width, matching what
  * [ImageEditorState.drawThickness] expects.
  */
-enum class BrushTool(val minThickness: Float, val maxThickness: Float) {
+enum class BrushTool(val minThickness: Float, val maxThickness: Float, val alpha: Int = OPAQUE_ALPHA) {
   MARKER(0.01f, 0.05f),
-  HIGHLIGHTER(0.03f, 0.08f),
+  HIGHLIGHTER(0.03f, 0.08f, alpha = HIGHLIGHTER_ALPHA),
   BLUR(0.052f, 0.092f);
 
   fun thicknessAt(fraction: Float): Float = minThickness + (maxThickness - minThickness) * fraction
+
+  /** [color] with this tool's opacity applied. */
+  fun applyAlpha(color: Int): Int = ColorUtils.setAlphaComponent(color, alpha)
 }
 
 /**