gl-renderer: Silence maybe-uninitialized warning

libweston/gl-renderer.c: In function 'compress_bands':
libweston/gl-renderer.c:481:6: warning: 'merged' may be used
uninitialized in this function [-Wmaybe-uninitialized]
   if (!merged) {
         ^

Warning produced by GCC 5.3 and 6.1, with -Og.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>

libweston/gl-renderer.c

@@ -455,7 +455,7 @@ static int
 compress_bands(pixman_box32_t *inrects, int nrects,
 		   pixman_box32_t **outrects)
 {
-	bool merged;
+	bool merged = false;
 	pixman_box32_t *out, merge_rect;
 	int i, j, nout;