[RFC] Extract line_consumption.c from misc1.c

This pull request extracts some functions that are concerned with physical line consumption of a buffer on screen from `misc1.c` and puts it in a file called `line_consumption.c`. (See #209)
Don't we need all functions to be pure, that will be called from this function?
Call chain: `plines_win -> diff_check_fill -> diff_check -> ex_diffupdate -> diff_clear -> free`