Return position information for pixel coordinates X and Y.
By default, X and Y are relative to text area of the selected window.
Optional third arg FRAME-OR-WINDOW non-nil specifies frame or window.
If optional fourth arg WHOLE is non-nil, X is relative to the left
edge of the window.
The return value is similar to a mouse click position:
(WINDOW AREA-OR-POS (X . Y) TIMESTAMP OBJECT POS (COL . ROW)
IMAGE (DX . DY) (WIDTH . HEIGHT))
The `posn-' functions access elements of such lists.
(fn X Y &optional FRAME-OR-WINDOW WHOLE)