Properly fix cursor position in Korean IME

[Susko3]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Description

• This PR improves on and fixes issues in fix Korean IME caret position #15643

If an IME doesn't provide a cursor position via GCS_CURSORPOS, the cursor position will now default to the end of the composition. This is what other programs on Windows do for Korean IME. (Sidenote: we could theoretically report the position as not set, as allowed by https://wiki.libsdl.org/SDL3/SDL_TextEditingEvent, but that can be done in another PR).

Existing Issue(s)

• Closes SDL_EVENT_TEXT_EDITING selection position is incorrectly reported for Microsoft Korean IME #15589

[slouken]

This looks good. Merged, thanks!