Fix bug #2071, alpha issue of DataSets

[qinidea]

PR Checklist:

• I have tested this extensively and it does not break any existing behavior.
• I have added/updated examples and tests for any new behavior.
• If this is a significant change, an issue has already been created where the problem / solution was discussed: [N/A, or add link to issue here]
  

PR Description

Issue #2071 has marked closed at 2018 but actually it still open.

LineChartRenderer calls c.drawBitmap with the mRenderPaint , which still contains the alpha from the last drawn dataset.

[hannesa2]

I guess, I've already included it https://github.com/AppDevNext/AndroidChart/blob/268c1b4034e5d397afae60f778930ab92bf30c62/chartLib/src/main/kotlin/info/appdev/charting/renderer/LineChartRenderer.kt#L85