Fix compilation with desktop GL
glDeleteFramebuffers() cannot be called directly for desktop GL where we link to the GL library dynamically. It is also only used from the ES2 code. Instead of adding additional #ifdef for it, we just move it to grabTexture(), since deleting it from the destructor, which is called from a different thread, is not valid anyway. It means we will create and destroy the ID every frame we capture, but since it is just registering an ID, this should be fine.