use main buffer and copy data to fbo texture (opengl)

[juliapaci]

NEEDS REVIEW

continuation of #5037
resolves #4729

renders all shaders to the default buffer and then copies it to the designated custom shader texture.

this is a draft pr because:

• it introduces a new shader "pipeline" which doesnt fit in with how the system was designed to work (which is only rendering to the fbo)
• im not sure if this is the best way to achieve shaders being able to sample their output while also drawing to the screen. the cusom fbo (previous implementation) was useful in that it modularized the custom shader stage in rendering

[Talljoe]

The latest change has a problem:

By getting rid of the fbo it looks like the shaders don't pick up the terminal text and just rewrite over themselves, hall-of-mirrors style.

This is what the previous commit looks like:

[juliapaci]

The latest change has a problem...

sorry about that. i happened to test on shaders that filled the entire screen so i didnt catch it.