[0.4.x] libvisual-plugins: Make actor "lv_scope" show silence as a solid line

[hartwork]

No description provided.

[kaixiong]

@hartwork Nice idea.

With better names, the code is clearer e.g.

const int max_displacement = video->height / 4;
...
const int y_origin = video->height / 2;
...
const int y_tip = y_origin + pcmbuf[(i >> 1) % PCM_SIZE] * max_displacement;
...

[hartwork]

@hartwork Nice idea.

With better names, the code is clearer e.g.

const int max_displacement = video->height / 4;
...
const int y_origin = video->height / 2;
...
const int y_tip = y_origin + pcmbuf[(i >> 1) % PCM_SIZE] * max_displacement;
...

@kaixiong true, applied

[kaixiong]

@hartwork The multiplication used to calculate the pixel buffer index is horrifying to me but I guess we'll optimize it in another PR.