Remove SDL_GL/Metal/Vulkan_GetDrawableSize().

SDL_GetWindowSizeInPixels supersedes those functions.

test/testgles2.c

@@ -539,7 +539,7 @@ render_window(int index)
         return;
     }
 
-    SDL_GL_GetDrawableSize(state->windows[index], &w, &h);
+    SDL_GetWindowSizeInPixels(state->windows[index], &w, &h);
     Render(w, h, &datas[index]);
     SDL_GL_SwapWindow(state->windows[index]);
     ++frames;
@@ -792,7 +792,7 @@ int main(int argc, char *argv[])
             /* Continue for next window */
             continue;
         }
-        SDL_GL_GetDrawableSize(state->windows[i], &w, &h);
+        SDL_GetWindowSizeInPixels(state->windows[i], &w, &h);
         ctx.glViewport(0, 0, w, h);
 
         data = &datas[i];