Improve computation of tetris difference

[timotheeg]

The tetris difference is presented like this:

And is computed as follow:

score_diff / tetris_value_at_level_lagging_player

It is dynamic in the sense that once the lagging player's level increases, the difference in number of tetrises will be updated accordingly.

That's not good enough though.

Take this setup for example:

• player 1 at 600k level 19
• player 2 at 450k, and 120 lines level 18

A tetris at level 18 is worth 22,800, so the difference is currently displayed as:

(600000-500000) / 22800 ~= 6.6T

BUT, in reality, the difference is not 6.6 tetris at the tetris value of level 18, since player 2 can only score 3 tetris at level 18 before transition to level19.

This information is known and could be use to run a smarter computation. Taking into account the level increases as tetrises (4 lines) are cleared.