Switch header convention from #include "SDL.h" to #include <SDL3/SDLh>

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

src/render/SDL_d3dmath.c

@@ -21,7 +21,6 @@
 #include "../SDL_internal.h"
 
 #if (SDL_VIDEO_RENDER_D3D || SDL_VIDEO_RENDER_D3D11 || SDL_VIDEO_RENDER_D3D12) && !SDL_RENDER_DISABLED
-#include "SDL_stdinc.h"
 
 #include "SDL_d3dmath.h"