Return the horizontal position of point. Beginning of line is column 0.
This is calculated by adding together the widths of all the displayed
representations of the character between the start of the previous line
and point (e.g., control characters will have a width of 2 or 4, tabs
will have a variable width).
Ignores finite width of frame, which means that this function may return
values greater than (frame-width).
Whether the line is visible (if `selective-display' is t) has no effect;
however, ^M is treated as end of line when `selective-display' is t.
Text that has an invisible property is considered as having width 0, unless
`buffer-invisibility-spec' specifies that it is replaced by an ellipsis.
(fn)