Support for BCSTM files in themes

[tasken]

Themes can now use bgm.bcstm for menu music: 3DS music files, mono or stereo, up to 48 kHz. Loop points stored in the file are used, so a track repeats past its intro instead of restarting. bgm.wav works exactly as before, and plays when a theme has no usable bgm.bcstm.

WAV handling moved into wavsource.cpp, and thememusic.cpp now just picks the source and keeps the same buffering.

Tested on a DSi using an Ace3DS+ and a DSpico.

[coderkei]

Whats the performance like?

I did test adding BCSTM support in the past but it would have clipping & slowdown in NTR mode (it also occurred in TWL mode to a lesser extent) since pre-calico DKA doesn't have proper threading like BlocksDS & newer Calico devkitarm does and Pico-Launcher's implementation relies on threading.

[tasken]

Decoding runs on the main loop without extra threads and buffers smoothly. Here's a demo on my DSi via an Ace3DS+ clone:

PXL_20260918_012932116.TS-10mb.mp4

Tested audio across various themes, and all of these work smoothly:

1. Stereo, 48,000 Hz, 55.90 s, ~439 kbps (attached)
2. Stereo, 44,100 Hz, 2.31 s, ~405 kbps
3. Mono, 48,000 Hz, 31.45 s, ~219 kbps
4. Stereo, 32,728 Hz, 8.65 s, ~299 kbps
5. Stereo, 24,000 Hz, 120.60 s, ~219 kbps

[coderkei]

Thanks, tested it out and all looks good to go