bugfix: ensure Cursor_line is always on a text line

Manual test used here:

  abc
  ```lines
  {"p1":{"y":72,"x":82},"mode":"line","p2":{"y":29,"x":169}}
  ```
  def
  ```lines
  {"p1":{"y":36,"x":56},"mode":"line","p2":{"y":59,"x":163}}
  ```
  ```lines
  ```
  ghi
  jkl

Hitting page-down moves the cursor from abc to ghi. The 'ghi' line
should be fully visible on screen.

3 files changed, 45 insertions, 3 deletions

diff --git a/README.md b/README.md
index 7c6ddf7..240b9a6 100644
--- a/README.md
+++ b/README.md
@@ -3,3 +3,11 @@ Known issues:
   On Linux, drags using the light touch get interrupted when a key is pressed.
   You'll have to press down to drag.
 * No support yet for Unicode graphemes spanning multiple codepoints.
+* The text cursor will always stay on the screen. This can have some strange
+  implications:
+    * A long series of drawings will get silently skipped when you hit
+      page-down, until a line of text can be showed on screen.
+    * If there's no line of text at the bottom of the file, one will be
+      created.
+  So far this app isn't really designed for all-drawing files. I'm really just
+  targeting mostly-text files with a few drawings mixed in.
diff --git a/main.lua b/main.lua
index a8cfc57..a23f426 100644
--- a/main.lua
+++ b/main.lua
@@ -172,14 +172,14 @@ function keychord_pressed(chord)
     Screen_top_line = Screen_bottom_line
     Cursor_line = Screen_top_line
     Cursor_pos = 1
+    Text.move_cursor_down_to_next_text_line_while_scrolling_again_if_necessary()
   elseif chord == 'pageup' then
     -- duplicate some logic from love.draw
     local y = Screen_height
     while y >= 0 do
       if Screen_top_line == 1 then break end
-      if Lines[Screen_top_line].mode == 'text' then
-        y = y - 15*Zoom
-      else
+      y = y - 15*Zoom
+      if Lines[Screen_top_line].mode == 'drawing' then
         y = y - Drawing.pixels(Lines[Screen_top_line].h)
       end
       Screen_top_line = Screen_top_line - 1
@@ -188,6 +188,7 @@ function keychord_pressed(chord)
       Cursor_pos = 1
     end
     Cursor_line = Screen_top_line
+    Text.move_cursor_down_to_next_text_line_while_scrolling_again_if_necessary()
   else
     Text.keychord_pressed(chord)
   end
diff --git a/text.lua b/text.lua
index f7ffb63..af205b6 100644
--- a/text.lua
+++ b/text.lua
@@ -155,6 +155,39 @@ function Text.keychord_pressed(chord)
   end
 end
 
+function Text.move_cursor_down_to_next_text_line_while_scrolling_again_if_necessary()
+  while Cursor_line <= #Lines do
+    if Lines[Cursor_line].mode == 'text' then
+      break
+    end
+    Cursor_line = Cursor_line + 1
+  end
+  -- hack: insert a text line at bottom of file if necessary
+  if Cursor_line > #Lines then
+    assert(Cursor_line == #Lines+1)
+    table.insert(Lines, {mode='text', data=''})
+  end
+  if Cursor_line > Screen_bottom_line then
+    Screen_top_line = Cursor_line
+    Text.scroll_up_while_cursor_on_screen()
+  end
+end
+
+function Text.scroll_up_while_cursor_on_screen()
+  local y = Screen_height - 15*Zoom -- for Cursor_line
+  while true do
+    if Screen_top_line == 1 then break end
+    y = y - 15*Zoom
+    if Lines[Screen_top_line].mode == 'drawing' then
+      y = y - Drawing.pixels(Lines[Screen_top_line].h)
+    end
+    if y < 15*Zoom then
+      break
+    end
+    Screen_top_line = Screen_top_line - 1
+  end
+end
+
 function Text.in_line(line, x,y)
   if line.y == nil then return false end  -- outside current page
   return x >= 16 and y >= line.y and y < line.y+15*Zoom