Implement wp_single_pixel_buffer_v1 protocol

This protocol allows clients to create single-pixel RGBA buffers. Now
that we have proper support for these buffers internally within Weston,
we can expose them to clients.

This bumps the build container version, as we now depend on
wayland-protocols v1.26.

Signed-off-by: Daniel Stone <daniels@collabora.com>

include/libweston/libweston.h

@@ -1318,6 +1318,10 @@ struct weston_compositor {
 	bool warned_about_unmapped_surface_or_view;
 };
 
+struct solid_buffer_values {
+	float r, g, b, a;
+};
+
 struct weston_buffer {
 	struct wl_resource *resource;
 	struct wl_signal destroy_signal;
@@ -1334,9 +1338,7 @@ struct weston_buffer {
 		struct wl_shm_buffer *shm_buffer;
 		void *dmabuf;
 		void *legacy_buffer;
-		struct {
-			float r, g, b, a;
-		} solid;
+		struct solid_buffer_values solid;
 	};
 
 	int32_t width, height;