improve behavior of SDL_SetHint(WithPriority)

[pionere]

Scenario 1. (overwrite hint temporary)

char* hint = SDL_GetHint(HINT_A);
hint = hint == NULL ? NULL : strdup(hint); // preserve previous hint
SDL_SetHint(HINT_A, "temp");               // overwrite hint
...
SDL_SetHint(HINT_A, hint);                 // restore hint
SDL_free(hint);
char* result = SDL_GetHint(HINT_A);

X	Result
expected	hint
currently	hint == NULL ? "temp" : hint;
with PR	hint

Scenario 2. (force suppress hint)

char* hint = SDL_GetHint(HINT_A);
SDL_SetHintWithPriority(HINT_A, NULL, SDL_HINT_OVERRIDE); // suppress hint (and env)
char* result = SDL_GetHint(HINT_A);

X	Result
expected	NULL
currently	env == NULL ? (hint == NULL ? NULL : hint) : env;
with PR	NULL

Scenario 3. (force suppress hint temporary)

char* hint = SDL_GetHint(HINT_A);
SDL_SetHintWithPriority(HINT_A, "temp", SDL_HINT_OVERRIDE);
...
SDL_SetHintWithPriority(HINT_A, NULL, SDL_HINT_OVERRIDE);
char* result = SDL_GetHint(HINT_A);

X	Result
expected	env ? env : NULL
currently	"temp"
with PR	NULL

To solve the last case, an explicit DelHint(WithPriority) function would be necessary...

[sezero]

Is this eligible for 2.0.22 (reviewed?) or should wait for later?

[slouken]

We need to decide whether a NULL value means clear the existing override or override with NULL. Here is a related bug: #5120

[slouken]

@icculus, can you review this now that #5120 is fixed?

[slouken]

After discussion, we agreed that setting a NULL value means overriding with NULL, and we're adding a new API SDL_ResetHint() to take care of the ambiguity.