log: Add some diagnostics to the log

Just a few useful things to get this started: egl vendor and extensions,
gles2 extensions, drm and kms info and just a bit of x11 backend info.
diff --git a/src/compositor-drm.c b/src/compositor-drm.c
index 23b814c..45ef6ef 100644
--- a/src/compositor-drm.c
+++ b/src/compositor-drm.c
@@ -1018,6 +1018,8 @@
 		return -1;
 	}
 
+	weston_log("using %s\n", filename);
+
 	ec->drm.fd = fd;
 	ec->gbm = gbm_create_device(ec->drm.fd);
 	ec->base.display = eglGetDisplay(ec->gbm);
@@ -1350,6 +1352,12 @@
 	output->base.set_dpms = drm_set_dpms;
 	output->base.switch_mode = drm_output_switch_mode;
 
+	weston_log("kms connector %d, crtc %d at mode %dx%d@%.1f\n",
+		   output->connector_id, output->crtc_id,
+		   output->base.current->width,
+		   output->base.current->height,
+		   output->base.current->refresh / 1000.0);
+
 	return 0;
 
 err_surface:
@@ -1748,6 +1756,8 @@
 	struct wl_event_loop *loop;
 	uint32_t key;
 
+	weston_log("initializing drm backend\n");
+
 	ec = malloc(sizeof *ec);
 	if (ec == NULL)
 		return NULL;