Replace non-standard Variable-Length Arrays with std::vector in Visualizer - #654

I accidentally included the commit for my other PR in this one, so I had to remake it, sorry.

In Visualizer::update() with stereo mode enabled, the code uses C99 Variable-Length Arrays (VLAs) to allocate audio buffers on the stack:

auto chan_samples = m_rendered_samples.size() / 2;
int16_t buf_left[chan_samples], buf_right[chan_samples];

VLAs aren't part of the C++ standard. Buffer sizes range from ~29 KB per buffer (wave modes) to ~57 KB per buffer (spectrum mode), totaling ~114 KB for both. This approaches or exceeds default stack limits on some platforms (256 KB on Windows, smaller on embedded), potentially causing silent stack overflow crashes.

Using std::vector instead allocates on the heap and is standard C++:

std::vector<int16_t> buf_left(chan_samples), buf_right(chan_samples);

Pass buffers to drawing functions using .data():

(this->*drawStereo)(buf_left.data(), buf_right.data(), chan_samples, half_height);

Also adds #include <vector>.