refactor(screen): screen.c delenda est

drawscreen.c vs screen.c makes absolutely no sense.
The screen exists only to draw upon it, therefore helper functions
are distributed randomly between screen.c and the file that
does the redrawing. In addition screen.c does a lot of drawing on the
screen.

It made more sense for vim/vim as our grid.c is their screen.c

Not sure if we want to dump all the code for option chars into
optionstr.c, so keep these in a optionchar.c for now.

src/nvim/drawscreen.h

@@ -22,6 +22,11 @@ enum {
 /// ('lines' and 'rows') must not be changed.
 EXTERN bool updating_screen INIT(= 0);
 
+EXTERN match_T screen_search_hl INIT(= { 0 });       // used for 'hlsearch' highlight matching
+
+#define W_ENDCOL(wp)   ((wp)->w_wincol + (wp)->w_width)
+#define W_ENDROW(wp)   ((wp)->w_winrow + (wp)->w_height)
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "drawscreen.h.generated.h"
 #endif