Returns the offset in bytes from the start of the current line returned via get_end_line.
This value may change in future invocations of this function if this is not yet parsed completely. This will
for example happen in the GtkCssProvider:
GtkCssSection:parsing-error signal. The end position and line may be identical to the
start position and line for sections which failed to parse anything successfully.
the offset in bytes from the start of the line.