Clean up #includes

Remove unused headers, and avoid relying on transitive inclusions

`include-what-you-use` has been very useful for this!

src/gfx/process.cpp

@@ -5,13 +5,9 @@
 #include <algorithm>
 #include <assert.h>
 #include <errno.h>
-#include <fstream>
 #include <inttypes.h>
-#include <limits.h>
-#include <memory>
 #include <optional>
 #include <png.h>
-#include <setjmp.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <string.h>