I ran this script in the include directory:
```sh
sed -i '' -e 's,#include "\(SDL.*\)",#include <SDL3/\1>,' *.h
```
I ran this script in the src directory:
```sh
for i in ../include/SDL3/SDL*.h
do hdr=$(basename $i)
if [ x"$(echo $hdr | egrep 'SDL_main|SDL_name|SDL_test|SDL_syswm|SDL_opengl|SDL_egl|SDL_vulkan')" != x ]; then
find . -type f -exec sed -i '' -e 's,#include "\('$hdr'\)",#include <SDL3/\1>,' {} \;
else
find . -type f -exec sed -i '' -e '/#include "'$hdr'"/d' {} \;
fi
done
```
Fixes https://github.com/libsdl-org/SDL/issues/6575
Most of these are probably harmless, but the changes to SDL_immdevice.c and SDL_pixels.c appear to have fixed genuine bugs.
SDL_audiocvt.c: By separating the calculation of the divisor, I got rid of the suspicion that dividing a double by an integer led to loss of precision.
SDL_immdevice.c: Added a missing test, one that could have otherwise led to dereferencing a null pointer.
SDL_events.c, SDL_gamecontroller.c, SDL_joystick.c, SDL_malloc.c, SDL_video.c: Made it clear the return values weren't used.
SDL_hidapi_shield.c: The size is zero, so nothing bad would have happened, but the SDL_memset() was still being given an address outside of the array's range.
SDL_dinputjoystick.c: Initialize local data, just in case IDirectInputDevice8_GetProperty() isn't guaranteed to write to it.
SDL_render_sw.c: drawstate.viewport could be null (as seen on line 691).
SDL.c: SDL_MostSignificantBitIndex32() could return -1, though I don't know if you want to cope with that (what I did) or SDL_assert() that it can't happen.
SDL_hints.c: Replaced boolean tests on pointer values with comparisons to NULL.
SDL_pixels.c: Looks like the switch is genuinely missing a break!
SDL_rect_impl.h: The MacOS static checker pointed out issues with the X comparisons that were handled by assertions; I added assertions for the Y comparisons.
SDL_yuv.c, SDL_windowskeyboard.c, SDL_windowswindow.c: Checked error-result returns.
Otherwise the thread might block for a long time (more than 10 seconds!).
It's not clear to me why this happens, or why its safe to do this with a
resource that's still in use, but we have, until recently, always
disposed of the AudioQueue first, so changing back is probably okay.
Also changed the disposal to allow in-flight buffers to reach hardware;
otherwise you lose the last little bit of audio that's already been queued
but not played, which you can hear clearly in the loopwave test program.
Fixes#6377.
Unopened devices, if removed, now send SDL_AUDIODEVICEREMOVED events with
a `which` field set to zero. Apps can use this to decide if they need to
refresh a list of devices being shown in an options menu, etc.
It's safe to call SDL_CloseAudioDevice(0), so even if they try to clean
up this bogus id, it should be safe.
Fixes#5199.
The 5.1 versions didn't use the new algorithm, and making that new
algorithm work took so many permutes that it was significantly slower
than just using the scalar versions.
However, mono-to-stereo is an extremely common conversion, and it's
trivial to accelerate it with plain SSE, so that was added!
