remove pixel copy in opengl backend

[Antoine Martin]

As identified in #1953, we call clone_plane via img.clone_pixel_data() in the opengl backend.

This was done back in r8069 to fix some tricky compatibility issues with older versions of pyopengl / python (see #717). See also r8111.

We should now be able to remove the extra pixel copy when zerocopy is supported. (new python, new pyopengl with match pyopengl-accelerate)

[Antoine Martin]

Done in r20370.

r20371 adds an environment variable to revert to the slower clone pixel mode:

XPRA_OPENGL_FORCE_CLONE=1 xpra attach ...

@maxmylyn: FYI, if I got this wrong then this may cause opengl rendering crashes.

[J. Max Mena]

Noted and closing.

I've checked this on my OpenGL enabled machines and everything works fine.