compositor: make keep_buffer a bool

The comments already call it bool, so let's just make it one

Reviewed-by: Bryce Harrington <b.harrington@samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/src/compositor-drm.c b/src/compositor-drm.c
index 07b83a7..bee41ef 100644
--- a/src/compositor-drm.c
+++ b/src/compositor-drm.c
@@ -1106,9 +1106,9 @@
 		    (es->buffer_ref.buffer &&
 		    (!wl_shm_buffer_get(es->buffer_ref.buffer->resource) ||
 		     (ev->surface->width <= 64 && ev->surface->height <= 64))))
-			es->keep_buffer = 1;
+			es->keep_buffer = true;
 		else
-			es->keep_buffer = 0;
+			es->keep_buffer = false;
 
 		pixman_region32_init(&surface_overlap);
 		pixman_region32_intersect(&surface_overlap, &overlap,