* README: Document changes.
* compositor.h (struct _BufferFuncs): Rename `get_render_device'
to `get_render_devices' and make it return a list of providers.
* dmabuf.c (MakeFeedback): Send 1 tranche for each device.
(InitDrmDevice): Rename to InitDrmDevices. Set up for multiple
devices.
(WriteFormatTable): Adjust call accordingly.
* egl.c (GetRenderDevice): Remove function.
(GetRenderDevices): New function.
(egl_buffer_funcs): Change accordingly.
* output.c (XLInitRROutputs): Require RandR 1.4.
* picture_renderer.c (GetRenderDevice): Remove calls to open.
(GetRenderDevices): New function.
(picture_buffer_funcs): Adjust accordingly.
* renderer.c (RenderGetRenderDevice): Delete function.
(RenderGetRenderDevices): New function.
* compositor.h (struct _RoleFuncs)
(struct _XdgRoleImplementationFuncs): New role functions
`outputs_changed'.
* output.c (NoticeOutputsMaybeChanged): Call surface
outputs_changed hook if it exists.
(XLGetMaxOutputBounds): New function.
* xdg_surface.c (OutputsChanged): New function.
(XLGetXdgSurface): Add new role function.
* xdg_toplevel.c (CurrentWindowGeometry): New function.
(SendStates): Factor out geometry calculation to that function.
(SendOutputBounds): New function.
(Commit, OutputsChanged): Send output bounds whenever
appropriate.
(XLGetXdgToplevel): Add new impl func.