Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

examples/game/01-snake/snake.c

@@ -94,7 +94,7 @@ static int are_cells_full_(SnakeContext *ctx)
 
 static void new_food_pos_(SnakeContext *ctx)
 {
-    while (SDL_TRUE) {
+    while (true) {
         const char x = (char) SDL_rand(SNAKE_GAME_WIDTH);
         const char y = (char) SDL_rand(SNAKE_GAME_HEIGHT);
         if (snake_cell_at(ctx, x, y) == SNAKE_CELL_NOTHING) {