sixel: change default max size to 10000x10000

It used to be the size of the window. This caused images to be cropped
when the application emitting them didn’t change the max size.

pgo/pgo.c

@@ -13,6 +13,7 @@
 
 #include "async.h"
 #include "config.h"
+#include "sixel.h"
 #include "user-notification.h"
 #include "vt.h"
 
@@ -200,7 +201,12 @@ main(int argc, const char *const *argv)
         .delayed_render_timer = {
             .lower_fd = lower_fd,
             .upper_fd = upper_fd
-        }
+        },
+        .sixel = {
+            .palette_size = SIXEL_MAX_COLORS,
+            .max_width = SIXEL_MAX_WIDTH,
+            .max_height = SIXEL_MAX_HEIGHT,
+        },
     };
 
     tll_push_back(wayl.terms, &term);