new globals: draw partial screen line up top

I'm not setting these yet. Rendering seems to be working after manually
setting them.

main.lua

@@ -47,6 +47,8 @@ Cursor_x, Cursor_y = 0, 0  -- in pixels
 -- scrolling support
 Screen_top_line = 1
 Screen_bottom_line = 1
+Top_screen_line_starting_pos = 1  -- when top of screen starts in between a wrapped line
+Bottom_screen_line_starting_pos = 1  -- when bottom of screen starts in between a wrapped line
 
 Current_drawing_mode = 'line'
 Previous_drawing_mode = nil
@@ -66,6 +68,7 @@ function love.load(arg)
   Screen_height = Screen_height-100
   love.window.setMode(Screen_width, Screen_height)
   love.window.setTitle('Text with Lines')
+--?   Line_width = 100
   Line_width = math.floor(Screen_width/2/40)*40
   love.keyboard.setTextInput(true)  -- bring up keyboard on touch screen
   love.keyboard.setKeyRepeat(true)

text.lua

@@ -19,7 +19,9 @@ function Text.draw(line, line_width, line_index)
     local frag_width = math.floor(frag_text:getWidth()*Zoom)
     if x + frag_width > line_width then
       assert(x > 25)  -- no overfull lines
-      y = y + math.floor(15*Zoom)
+      if line_index > Screen_top_line or pos >= Top_screen_line_starting_pos then
+        y = y + math.floor(15*Zoom)
+      end
       x = 25
       if line.screen_line_starting_pos == nil then
         line.screen_line_starting_pos = {1, pos}
@@ -27,7 +29,9 @@ function Text.draw(line, line_width, line_index)
         table.insert(line.screen_line_starting_pos, pos)
       end
     end
-    love.graphics.draw(frag_text, x,y, 0, Zoom)
+    if line_index > Screen_top_line or pos >= Top_screen_line_starting_pos then
+      love.graphics.draw(frag_text, x,y, 0, Zoom)
+    end
     -- render cursor if necessary
     local frag_len = utf8.len(frag)
     if line_index == Cursor_line then