Removed speculative comment

Directly setting the velocity on solid boundaries is necessary. You
never explicitly calculate the pressure of the solid wall, and it's
different for each face anyways, so there's no single value to store.

main.cxx

@@ -658,7 +658,7 @@ void project(float dt, float u[Y][X], float v[Y][X], float uout[Y][X], float vou
   // update horizontal velocities
   for (size_t y = 0; y < Y; ++y) {
     for (size_t x = 0; x < X-1; ++x) {
-      if (g_solid[y][x] || g_solid[y][x+1]) { // todo: unnecessary? probably same as !is_water
+      if (g_solid[y][x] || g_solid[y][x+1]) {
         uout[y][x] = 0.f;
       } else if (is_water(y,x) || is_water(y,x+1)) {
         uout[y][x] = u[y][x] - k_invd * k_invs * dt * (p[y][x+1] - p[y][x]);