Fix BrightnessSlider transparent background

After a thorough refactoring of the BrightnessSlider somewhere after
release 1.0.4, the slider background would falsely show a gradient going
from full transparency to zero transparency.

This commit removes the alpha logic and replaces it with the previously
used shader going from pure black to the (pure) selected colour.

Fixes #42

colorpicker-compose/src/main/kotlin/com/github/skydoves/colorpicker/compose/BrightnessSlider.kt

@@ -168,8 +168,8 @@ public fun BrightnessSlider(
         canvas.drawImage(it, Offset.Zero, Paint())
 
         // draw a linear gradient color shader.
-        val startColor = controller.pureSelectedColor.value.copy(alpha = 0f)
-        val endColor = controller.pureSelectedColor.value.copy(alpha = 1f)
+        val startColor = Color.Black
+        val endColor = controller.pureSelectedColor.value
         val shader = LinearGradientShader(
           colors = listOf(startColor, endColor),
           from = Offset.Zero,