destroy all textures on window destruction and restore OpenGL objects when Android resumes the app

engine/texture.cpp

@@ -25,6 +25,11 @@ Texture2D& TextureManager::get(uint16_t id)
     return m_textures[id];
 }
 
+void TextureManager::invalidate()
+{
+    m_textures.clear();
+}
+
 bool Texture2D::create(int width, int height, GLint internal_format, GLint format, const uint8_t* data)
 {
     m_width = width;