Fix usage of sizeof() in test/testgles*.c files

(cherry-picked from commit 71f2864b3a24e144eaf4ae6e86eee0dd5e9f89b1)

test/testgles.c

@@ -173,7 +173,7 @@ main(int argc, char *argv[])
         quit(2);
     }
 
-    context = (SDL_GLContext *)SDL_calloc(state->num_windows, sizeof(context));
+    context = (SDL_GLContext *)SDL_calloc(state->num_windows, sizeof(*context));
     if (context == NULL) {
         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Out of memory!\n");
         quit(2);

test/testgles2.c

@@ -612,7 +612,7 @@ main(int argc, char *argv[])
         return 0;
     }
 
-    context = (SDL_GLContext *)SDL_calloc(state->num_windows, sizeof(context));
+    context = (SDL_GLContext *)SDL_calloc(state->num_windows, sizeof(*context));
     if (context == NULL) {
         SDL_Log("Out of memory!\n");
         quit(2);

test/testgles2_sdf.c

@@ -522,7 +522,7 @@ main(int argc, char *argv[])
         return 0;
     }
 
-    context = (SDL_GLContext *)SDL_calloc(state->num_windows, sizeof(context));
+    context = (SDL_GLContext *)SDL_calloc(state->num_windows, sizeof(*context));
     if (context == NULL) {
         SDL_Log("Out of memory!\n");
         quit(2);