Merge OpenGL and Cairo Mobjects

[awesome-developer-20]

Problem:
Maintaining two separate mobject trees is difficult and from user end to changing all constructor names from Tex to OpenGLTex is irritating to change rendering method.

Solution:
Merge Mobject and OpenGLMobject classes and separate out rendering logic in renderer.

[behackl]

We are already in the process of doing exactly that, via the metaclass approach introduced in #1272. :-)

[kilacoda-old]

We're planning to move away from cairo entirely in the near future.

[awesome-developer-20]

We're planning to move away from cairo entirely in the near future.

That really awesome!!

[awesome-developer-20]

Till when you are planning to completely move to opengl?

[behackl]

We don't have a concrete timeframe.

[awesome-developer-20]

It's okay, Thanks