screenshoter: Make recorder handle PIXMAN_x8r8g8b8

This is what the pixman renderer reports for the read format.
diff --git a/src/screenshooter.c b/src/screenshooter.c
index dde052f..d088cf7 100644
--- a/src/screenshooter.c
+++ b/src/screenshooter.c
@@ -465,6 +465,7 @@
 	header.magic = WCAP_HEADER_MAGIC;
 
 	switch (compositor->read_format) {
+	case PIXMAN_x8r8g8b8:
 	case PIXMAN_a8r8g8b8:
 		header.format = WCAP_FORMAT_XRGB8888;
 		break;