[idea] use a "copy" shader to solve some cases

[gre]

Here are the cases:

• support for width, height in the specific case of root GL.Node #33 surface size is different of root node size
• Cache a node: GL.Node saveBuffer+cached props #27 if you cache the root node, we need to keep the rendering from one frame to another

we might be able to do this from gl-react. Also after this, the width and height of the root node should probably lead more the GL Surface (if it's not already the case).

such a "copy" shader is probably not the most efficient idea, but i'm not seeing another way in OpenGL/WebGL. we need to be sure to only enable it if it's required.

[gre]

need to benchmark solving the resize with a copy shader VS solving it with UIView's transform

[gre]

(will still need copy shader for the second use-case though)

[gre]

just a reminder that later we will have to introduce the feature to set the interpolation of FBOs (so you can switch between linear and none)