Added GetPixelSpan() with offsets

[mattleibow]

Description of Change

Added some APIs to help get the span to an offset instead of only allowing the start of the pixmap memory.

Bugs Fixed

• Not a bug, but more the replacement for the GetAddr members we removed in 3.0

API Changes

// extensions
int SKColorType.GetBitShiftPerPixel ();

// members
Span<byte> SKBitmap.GetPixelSpan (int x, int y);
int SKImageInfo.BitShiftPerPixel { get; };
int SKImageInfo.GetPixelBytesOffset (int x, int y);
int SKPixmap.BitShiftPerPixel { get; };
int SKPixmap.GetPixelBytesOffset (int x, int y);
Span<byte> SKPixmap.GetPixelSpan (int x, int y);
Span<T> SKPixmap.GetPixelSpan<T> (int x, int y);

Behavioral Changes

None.

Required skia PR

None.

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Merged related skia PRs
• Changes adhere to coding standard
• Updated documentation