opengles2: Rework render targets.

Now this keeps one FBO for each SDL_TEXTUREACCESS_TARGET texture, and doesn't
reuse it. We check if the FBO is "complete" once, at creation time, and
setting a render target merely has to bind the right FBO and never look back.

This simplifies the code and removes a guaranteed pipeline stall when setting
a new render target.

Fixes #15524.