Cleanup includes

Improved manually with the help of neovim LSP warnings and iwyu:

    iwyu -Ibuilddir/app/ -Iapp/src/ app/src/XXX.c

app/src/display.c

@@ -1,6 +1,8 @@
 #include "display.h"
 
 #include <assert.h>
+#include <inttypes.h>
+#include <string.h>
 #include <libavutil/pixfmt.h>
 
 #include "util/log.h"