libweston: consolidate 'Using GL/Pixman renderer' log message

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(). Only print the message after
initializing the renderer has succeeded.

This effectively adds the message to the DRM, headless, and Wayland
backends.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>