compositor: prepare for multi-planar surfaces.

Make weston_surface::texture and ::surface an array, while keeping
[0] for RGB surfaces.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
diff --git a/src/compositor.h b/src/compositor.h
index 0d49dec..8e3225f 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -385,7 +385,8 @@
 struct weston_surface {
 	struct wl_surface surface;
 	struct weston_compositor *compositor;
-	GLuint texture;
+	GLuint textures[3];
+	int num_textures;
 	pixman_region32_t clip;
 	pixman_region32_t damage;
 	pixman_region32_t opaque;
@@ -446,7 +447,8 @@
 
 	struct wl_list frame_callback_list;
 
-	EGLImageKHR image;
+	EGLImageKHR images[3];
+	int num_images;
 
 	struct wl_buffer *buffer;
 	struct wl_listener buffer_destroy_listener;