libweston: consolidate 'Using GL/Pixman renderer' log message
This consolidates the 'Using GL/Pixman renderer' message emitted by the
PipeWire, RDP, VNC, and X11 backends by moving the weston_log()
into
weston_compositor_init_renderer()
. The message is only printed after
initializing the renderer has succeeded.
This effectively adds the message to the DRM, headless, and Wayland backends.