some screen functions don't accept fractional coordinates

[artfwo]

cairo supports fractional coordinates in between pixels, perhaps we should too. currently strict integer checking in some functions results in errors like this:

stack traceback:
    [C]: in function 's_move_rel'
    /home/pi/norns/lua/screen.lua:168: in function 's_text_center'
    /home/pi/norns/lua/screen.lua:121: in function 'screen.text_center'
    (...tail calls...)

affected functions:

• _screen_font_size
• _screen_move
• _screen_move_rel
• _screen_line_width
• _screen_line
• _screen_line_rel

[tehn]

agreed, i also ran into this yesterday while menu-fixing.