Move buffer allocation outside of x11 backend

[i509VCB]

Right now the X11 backend manages window creation, input, buffer allocation and presentation.

This works fine assuming all the user wants to present are Dmabufs. However for software rendered buffers, a different buffer types will be needed for presentation.

Along with this, the X11 backend will also expose what modifiers and formats are supported for presentation of dmabufs. And there will need to be a way to find the drm device. In the case of X11, that is done by name in EGL or DRI3's provided render/primary node.

Then in the future, a way to present with XShm will be added.