ui: disable clearing almost everywhere

Avoid clearing the screen in most situations. NOT_VALID should be
equivalent to CLEAR unless some external force messed up the terminal,
for these situations <c-l> and :mode will still clear the screen.

Also eliminate some obsolete code in screen.c, that dealt with that in
vim drawing window 1 can mess up window 2, but this never happens in
nvim.

But what about slow terminals? There is two common meanings in which
a terminal is said to be "slow":

Most commonly (and in the sense of vim:s nottyfast) it means low
bandwidth for sending bytes from nvim to the terminal. If the screen is
very similar before and after the update_screen(CLEAR) this change
should reduce bandwidth. If the screen is quite different, but there is
no new regions of contiguous whitespace, clearing doesn't reduce
bandwidth significantly. If the new screen contains a lot of whitespace,
it will depend of if vsplits are used or not: as long as there is no
vsplits, ce is used to cheaply clear the rest of the line, so
full-screen clear is not needed to reduce bandwith. However a left
vsplit currently needs to be padded with whitespace all the way to the
separator. It is possible ec (clear N chars) can be used to reduce
bandwidth here if this is a problem. (All of this assumes that one
doesn't set Normal guibg=... on a non-BCE terminal, if you do you are
doomed regardless of this change).

Slow can also mean that drawing pixels on the screen is slow. E-ink
screens is a recent example. Avoiding clearing and redrawing the
unchanged part of the screen will always improve performance in these
cases.

test/functional/eval/input_spec.lua

@@ -149,7 +149,7 @@ describe('input()', function()
       {EOB:~                        }|
       {T:Foo>}Bar^                  |
     ]])
-    command('redraw!')
+    command('mode')
     screen:expect{grid=[[
                                |
       {EOB:~                        }|
@@ -165,7 +165,7 @@ describe('input()', function()
       {EOB:~                        }|
       {T:Foo>}Ba^                   |
     ]])
-    command('redraw!')
+    command('mode')
     screen:expect{grid=[[
                                |
       {EOB:~                        }|
@@ -347,7 +347,7 @@ describe('inputdialog()', function()
       {EOB:~                        }|
       {T:Foo>}Bar^                  |
     ]])
-    command('redraw!')
+    command('mode')
     screen:expect{grid=[[
                                |
       {EOB:~                        }|
@@ -363,7 +363,7 @@ describe('inputdialog()', function()
       {EOB:~                        }|
       {T:Foo>}Ba^                   |
     ]])
-    command('redraw!')
+    command('mode')
     screen:expect{grid=[[
                                |
       {EOB:~                        }|