SDL2 audio synthesis.

Initially based on https://github.com/lundstroem/synth-samples-sdl2 example number 3. Started with this simple code and evolved it into a polyphonic synth.

I wanted more features and grabbed some audio code for filtering, compression, etc. from https://github.com/velipso/sndfilter (Thanks, Sean!)

Linux Ubuntu 20.04 builds with apt installed SDL & GLEW

Got Windows Build working with both:

1. VS Code using Microsoft CMake & Select "VS Community 2019 Release - amd64" kit output in "build"
2. VS 2019 Community Edition output in "out/build"

• [DONE] needs better ADSR envelope
• [DONE] one note at a time, needs polyphony
• [DONE] basic GUI
• [DONE] sine, square, triangle & sawtooth waves
• [DONE] could use stereo pan
• [DONE] needs a compressor
• [DONE] could use resonant LPF after synth
• low freq osc input to offset pitch, amplitude, phase
• adjust pitch indexes to match midi