Added the ability to go between a string offset and render position.

[slouken]

This allows things like implementing a cursor that automatically follows font size and text direction changes.

[1bsyl]

@slouken since you're in a middle of modification, there are a few shadow variables:

showfont.c:457:33: warning: declaration of ‘outline’ shadows a previous local [-Wshadow]
457 | int outline = TTF_GetFontOutline(font);
| ^~~~~~~
/showfont.c:128:9: note: shadowed declaration is here
128 | int outline;
| ^~~~~~~
showfont.c:501:35: warning: declaration of ‘ptsize’ shadows a previous local [-Wshadow]
501 | float ptsize = TTF_GetFontSize(font);
| ^~~~~~
showfont.c:118:11: note: shadowed declaration is here
118 | float ptsize;
| ^~~~~~
showfont.c:508:35: warning: declaration of ‘ptsize’ shadows a previous local [-Wshadow]
508 | float ptsize = TTF_GetFontSize(font);
| ^~~~~~
showfont.c:118:11: note: shadowed declaration is here
118 | float ptsize;

[sezero]

This breaks build if harfbuzz is disabled:

src/SDL_ttf.c: In function ‘TTF_OpenFontWithProperties’:
src/SDL_ttf.c:1922: error: ‘TTF_Font’ has no member named ‘hb_direction’
src/SDL_ttf.c:1922: error: ‘HB_DIRECTION_LTR’ undeclared (first use in this function)
src/SDL_ttf.c:1922: error: (Each undeclared identifier is reported only once
src/SDL_ttf.c:1922: error: for each function it appears in.)
src/SDL_ttf.c:1923: error: ‘TTF_Font’ has no member named ‘hb_script’
src/SDL_ttf.c:1923: error: ‘HB_SCRIPT_UNKNOWN’ undeclared (first use in this function)