This is essentially the same as was added in d95d2d70, but with clearer
error handling. It's implemented in a private header file so that it
can be shared with SDL_shape, which also wants this functionality.
Resolves: https://github.com/libsdl-org/SDL/issues/8319
Signed-off-by: Simon McVittie <smcv@collabora.com>
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
Sam Lantinga
Ryan C. Gordon
Simon McVittie
Anonymous Maarten