render: add WLR_RENDER_NO_EXPLICIT_SYNC env var

This can be handy to figure out if a bug is due to explicit sync.

docs/env_vars.md

@@ -12,6 +12,8 @@ wlroots reads these environment variables
   renderers: gles2, pixman, vulkan)
 * *WLR_RENDER_DRM_DEVICE*: specifies the DRM node to use for
   hardware-accelerated renderers.
+* *WLR_RENDER_NO_EXPLICIT_SYNC*: set to 1 to disable explicit synchronization
+  support in renderers.
 * *WLR_EGL_NO_MODIFIERS*: set to 1 to disable format modifiers in EGL, this can
   be used to understand and work around driver bugs.
 

render/wlr_renderer.c

@@ -274,6 +274,9 @@ out:
 	if (own_drm_fd && drm_fd >= 0) {
 		close(drm_fd);
 	}
+	if (renderer != NULL && env_parse_bool("WLR_RENDER_NO_EXPLICIT_SYNC")) {
+		renderer->features.timeline = false;
+	}
 	return renderer;
 }