Bump version number to 0.0.21.
Bump version number to 0.0.21.

ChrisBriggsy/Win2D,zjsxwc/Win2D,zjsxwc/Win2D,LanceMcCarthy/Win2D,ChrisBriggsy/Win2D,zjsxwc/Win2D,ChrisBriggsy/Win2D,LanceMcCarthy/Win2D,LanceMcCarthy/Win2D