Revert "skia: SkPaint::SkPaint(): don't cast away const"

This reverts commit d27af6b8a50f73804d9a3d60c1058e62a79d87bc.
Revert "Skia: Introduce caching of unscaled Bitmaps."

This reverts commit 84029822a690509c66bf2b0ed5d11f281375b904.