Get graphics context from extension

[aglitchman]

@Jay

I do think ReadPixels is a more complicated case though - because (I think) it always reads from the window frame-buffer, and it might be desirable to read from the context’s currently bound render target for some use-cases, so usage and interface there may need more thought, in order to be more flexible for such possible future cases.

Could you please write how you would use this in your extension and how the end user would end up using it? The simplest example.