stop recomputing Cursor_x now that we're saving it

Dependencies

• [2] C42QQZSF another piece of support for line wrapping in cursor up
• [3] 76AIXR7H bugfix
• [4] MGT5FTJ3 first stab at supporting wrapping in cursor up
• [5] 5ED3YGHW planning out cursor up/down over wrapped lines
• [6] 3CSIZJ33 clearer comments
• [7] XNFTJHC4 split keyboard handling between Text and Drawing
• [8] SVJZZDC3 snapshot - no, that's all wrong
• [*] BULPIBEG beginnings of a module for the text editor

Change contents

• edit in text.lua at line 239
  [3.358]→[3.358:437](∅→∅)
  − local old_x = Text.cursor_x(Lines[new_cursor_line].data, Cursor_pos)
• replacement in text.lua at line 241
  [2.70]→[2.70:151](∅→∅)
  − Cursor_pos = Text.nearest_cursor_pos(Lines[Cursor_line].data, old_x)
  [2.70]

  [2.151]
  + Cursor_pos = Text.nearest_cursor_pos(Lines[Cursor_line].data, Cursor_x)
• replacement in text.lua at line 253
  [2.507]→[2.507:595](∅→∅)
  − Cursor_pos = screen_line_starting_pos + Text.nearest_cursor_pos(s, old_x) - 1
  [2.507]

  [3.556]
  + Cursor_pos = screen_line_starting_pos + Text.nearest_cursor_pos(s, Cursor_x) - 1
• edit in text.lua at line 281
  [3.4681]→[3.4681:4758](∅→∅)
  − local old_x = Text.cursor_x(Lines[new_cursor_line].data, Cursor_pos)
• replacement in text.lua at line 282
  [3.4796]→[3.79:156](∅→∅)
  − Cursor_pos = Text.nearest_cursor_pos(Lines[Cursor_line].data, old_x)
  [3.4796]

  [3.4885]
  + Cursor_pos = Text.nearest_cursor_pos(Lines[Cursor_line].data, Cursor_x)