LibGfx: Make all image decoders reject image sizes above 16384 pixels

Let's just say no to shenanigans by capping images at 16384 pixels both
wide and tall. If a day comes in the future where we need to handle
images larger than this, we can deal with it then.

Libraries/LibGfx/PBMLoader.cpp

@@ -60,8 +60,8 @@ struct PBMLoadingContext {
     State state { State::NotDecoded };
     const u8* data { nullptr };
     size_t data_size { 0 };
-    int width { -1 };
-    int height { -1 };
+    size_t width { 0 };
+    size_t height { 0 };
     RefPtr<Gfx::Bitmap> bitmap;
 };