Use cls() instead of rectfill() to try and reduce 'black screen' errors.

debug.lua

@@ -16,7 +16,7 @@ function debug_t(mode)
       debug_map = not debug_map
    elseif mode == "sfx" then
       debug_sfx = not debug_sfx
-   elseif mode == "" then
+   elseif mode == "mod" then
       debug_mod_buffer = not debug_mod_buffer
    end
 end

main.lua

@@ -14,7 +14,7 @@ function _update()
 end
 
 function _draw()
-   rectfill(0, 0, 127, 127, 0)
+   cls()
    -- the screen is 128x128 pixels, so it fits 16x16 sprites
    map(camera_pos_x, camera_pos_y, 0, 0, 16, 16)
    draw_player()