EXAMPLES: lua: avoid screen refresh effect in "trisdemo"

In current version of the game, there is a "screen refresh" effect: the
screen is cleared before being re-drawn.
I moved the clear right after the connection is opened and removed it
from rendering time.

examples/lua/trisdemo.lua

@@ -112,7 +112,7 @@ local function rotate_piece(piece, piece_id, px, py, board)
 end
 
 function render(applet, board, piece, piece_id, px, py, score)
-    local output = clear_screen .. cursor_home
+    local output = cursor_home
     output = output .. game_name .. " - Lines: " .. score .. "\r\n"
     output = output .. "+" .. string.rep("-", board_width * 2) .. "+\r\n"
     for y = 1, board_height do
@@ -160,6 +160,7 @@ function handler(applet)
     end
 
     applet:send(cursor_hide)
+    applet:send(clear_screen)
 
     -- fall the piece by one line every delay
     local function fall_piece()