SDL API renaming: *Is* functions

Feedback from @icculus:
"IsTablet" uses "is" as a form of "to be" ...like, the actual question is of its nature.

The rest is just a superfluous word in the question and it flows as better English with if (RectEmpty) than if (IsRectEmpty)

Fixes https://github.com/libsdl-org/SDL/issues/6932

test/testhittesting.c

@@ -32,7 +32,7 @@ hitTest(SDL_Window *window, const SDL_Point *pt, void *data)
     int w, h;
 
     for (i = 0; i < numareas; i++) {
-        if (SDL_IsPointInRect(pt, &areas[i])) {
+        if (SDL_PointInRect(pt, &areas[i])) {
             SDL_Log("HIT-TEST: DRAGGABLE\n");
             return SDL_HITTEST_DRAGGABLE;
         }