Reduce popping during audio initialization using the additive DAC

[Nebuleon]

Description

The previous DAC initialization code caused two pops if AUDIO_DAC_OFF_VALUE was not 0 because the buffer sent to the DAC had one AUDIO_DAC_OFF_VALUE then the rest 0 before leaving the DAC again at AUDIO_DAC_OFF_VALUE:

static dacsample_t dac_buffer_empty[AUDIO_DAC_BUFFER_SIZE] = {AUDIO_DAC_OFF_VALUE}; /* other elements get initialized to 0 */

This commit corrects the initialization and eliminates this source of popping.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[Nebuleon]

I've gone and marked this as a draft because, while investigating why that empty buffer wasn't marked const, I found out that that buffer is also what gets used when the DAC is emitting samples. I'll review the whole initialization, because the way this code is written, there might be some shenanigans with stale data going on.

[Nebuleon]

Alright, now that's much better. I've made it so that the "empty" buffer, which is also used throughout the keyboard's runtime, is called dac_buffer rather than dac_buffer_empty, and moved its initialization such that, instead of being in .data, copied from flash into RAM, it's filled with AUDIO_DAC_OFF_VALUE whenever audio_driver_initialize gets called, so if for any reason the audio needs to be reinitialized, it will get a new empty buffer again. All the while taking 512 (AUDIO_DAC_BUFFER_SIZE * sizeof(dacsample_t)) fewer bytes of data and a few more bytes of code in flash to regenerate the whole thing. The RAM usage stays the same.