bugfix: crash when using mouse wheel

All the Text functions assume the cursor is always on a text line. I was
violating that invariant.

* When scrolling up, I start the cursor at the top-most line below the
  screen top.

* When scrolling down, I start the cursor at the top-most line below the
  screen bottom.

  I think it would feel slightly more natural for it to be the
  bottom-most line above the screen bottom.

  However, the Text functions maintain an invariant that the bottom-most
  line in a buffer will be text. There's no such invariant for the
  top-most line.

edit.lua

@@ -343,11 +343,13 @@ end
 function edit.mouse_wheel_move(State, dx,dy)
   if dy > 0 then
     State.cursor1 = {line=State.screen_top1.line, pos=State.screen_top1.pos}
+    edit.put_cursor_on_next_text_line(State)
     for i=1,math.floor(dy) do
       Text.up(State)
     end
   elseif dy < 0 then
     State.cursor1 = {line=State.screen_bottom1.line, pos=State.screen_bottom1.pos}
+    edit.put_cursor_on_next_text_line(State)
     for i=1,math.floor(-dy) do
       Text.down(State)
     end

source_edit.lua

@@ -333,11 +333,13 @@ end
 function edit.mouse_wheel_move(State, dx,dy)
   if dy > 0 then
     State.cursor1 = {line=State.screen_top1.line, pos=State.screen_top1.pos}
+    edit.put_cursor_on_next_text_line(State)
     for i=1,math.floor(dy) do
       Text.up(State)
     end
   elseif dy < 0 then
     State.cursor1 = {line=State.screen_bottom1.line, pos=State.screen_bottom1.pos}
+    edit.put_cursor_on_next_text_line(State)
     for i=1,math.floor(-dy) do
       Text.down(State)
     end