[FEATURE] Expose SkiaApi class

[kekekeks]

SkObject initialization and housekeeping is quite costly in both CPU and allocations. On hot code paths it's desirable to just deal with raw IntPtr's and Skia C API and manage those manually.

Performance benefits far outweigh the loss of object-oriented API and backwards compatibility with previous SkiaSharp versions.

Since SkObject already exposes Handle property the only change that is needed is to make SkiaApi class to be public rather than internal.

[daltonks]

I'd like this as well, especially for path iterators. This seems really expensive when it's applied to every path verb:

      public unsafe SKPathVerb Next(Span<SKPoint> points)
      {
        if (points == (Span<SKPoint>) (SKPoint[]) null)
          throw new ArgumentNullException(nameof (points));
        if (points.Length != 4)
          throw new ArgumentException("Must be an array of four elements.", nameof (points));
        fixed (SKPoint* points1 = &points.GetPinnableReference())
          return SkiaApi.sk_path_rawiter_next(this.Handle, points1);
      }