ofFbo working with ofTextures created externally.

ive added another variation of the createAndAttachTexture() method to ofFbo.
`void createAndAttachTexture(ofTexture & texture, GLenum internalFormat, GLenum attachmentPoint);`

this allows for ofTextures that have been created externally to be used inside the ofFbo.
this addition does not alter any existing ofFbo functionality.

this issue has been discussed before here #2557

@arturoc, can you let me know what you think of this?
@jvcleave, better late then never :)