Fix integer overflow panic on large display

[sylr]

$ zig-out/bin/DOOM-fire
Screen size: 375w x 89h

Standard colors:
▏  0  ▏  1  ▏  2  ▏  3  ▏  4  ▏  5  ▏  6  ▏  7
▏  8  ▏  9  ▏ 10  ▏ 11  ▏ 12  ▏ 13  ▏ 14  ▏ 15

216 colors:
 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87
 88 89 90 91 92 93 94 95 96 97 98 99100101102103104105106107108109110111112113114115116117118119120121122123
124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159
160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195
196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231

Grayscale:
▏232 ▏233 ▏234 ▏235 ▏236 ▏237 ▏238 ▏239 ▏240 ▏241 ▏242 ▏243 ▏244 ▏245 ▏246 ▏247 ▏248 ▏249 ▏250 ▏251 ▏252 ▏253 ▏254 ▏255

  How much is the fish?
             -- Scooter
Press return to continue...
thread 1068243 panic: integer overflow
/Users/sylvain/tmp/DOOM-fire-zig/src/main.zig:553:33: 0x104e7ab83 in showDoomFire (DOOM-fire)
    const FIRE_SZ: u16 = FIRE_H * FIRE_W;
                                ^
/Users/sylvain/tmp/DOOM-fire-zig/src/main.zig:695:17: 0x104e78d3b in main (DOOM-fire)
    showDoomFire();
                ^
/opt/homebrew/Cellar/zig/0.13.0/lib/zig/std/start.zig:524:37: 0x104e789d3 in main (DOOM-fire)
            const result = root.main() catch |err| {
                                    ^
???:?:?: 0x19b517153 in ??? (???)
???:?:?: 0xa04fffffffffffff in ??? (???)
[1]    68669 abort      zig-out/bin/DOOM-fire

[const-void]

Thank you! Great fix!