[feature] enable debug option to write bitmaps

[willbr]

I've enjoyed playing with the bitmap export feature in debug mode.

Maybe we should enable it by default?

https://github.com/foone/3DMMForever/blob/79b301091175459dd39ec53bbb8a711248e969c6/src/studio/utest.rc#L73

https://github.com/foone/3DMMForever/blob/79b301091175459dd39ec53bbb8a711248e969c6/kauai/src/mbmp.cpp#L766-L779

[willbr]

if you rename the files to bmp, ffmpeg can convert them to a gif

ffmpeg -r 6 -i cel%04d.bmp output.gif

Also, the files are saved in the VirtualStore for me on Windows 11 C:\Users\wjbr\AppData\Local\VirtualStore\Program Files (x86)\Microsoft Kids\USERS\wjbr