Fix BrightnessSlider transparent background

[moertel]

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. The previous logic, for reference:

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

Lines 172 to 177 in 0320c67

	val shader = LinearGradientShader(
	colors = listOf(Color.Black, controller.pureSelectedColor.value),
	from = Offset.Zero,
	to = Offset(bitmapSize.width.toFloat(), bitmapSize.height.toFloat()),
	tileMode = TileMode.Clamp,
	)

Fixes #42

[skydoves]

Looks good to me!