Allow server implemention to choose GL config

[Gerry Boland]

Mir's GL implementation appears to support only GLES2. On Desktops, it is more likely that Open GL is available and reliable. Qt expects OpenGL on the desktop, and only GLES2 on mobile devices.

QtMir currently has to let Mir choose a GL context suitable for it, and Mir allows it to specify depth & stencil buffers. Mir always returns a GLES2 context, and Qt just has to work with that. Thanks for MESA being forgiving, Qt can perform GL with this GLES2 context ok, most of the time.

But Qt should be able to specify GL or GLES2 at least, so it can get a proper OpenGL context.

Qt has a lot of internal logic to choose a correct context for itself.
http://code.qt.io/cgit/qt/qtbase.git/tree/src/platformsupport/eglconvenience/qeglconvenience_p.h?h=dev#n75
see the "chooseConfig" method of the QEGLConfigChooser.

It would be ideal if Mir supported a callback that could allow the third party to choose the correct config for its renderer, as opposed to Mir doing that.

[Gerry Boland (gerboland)]

I suppose if an OpenGL context is chosen, Mir would need to modify it's own renderer to suit - qtmir does rely on Mir's gTextureSource::l_bind_to_texture to work

[Daniel van Vugt (vanvugt)]

Thanks. Duplicate of bug 1420581