Simplify DLL hook mechanism

This change deletes the GetProcAddress hook and exports symbols
corresponding to the hooked functions from each hook DLL instead;
we stop at redirecting LoadLibrary/GetModuleHandle calls to the
hook DLL. This simplified approach has less hidden magic going on
behind the scenes and is more readily composable (i.e. a hook DLL
can export redirect symbols for more than one dynamically-loaded
DLL).

hooklib/gfx.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <windows.h>
+
 #include <stdbool.h>
 
 struct gfx_config {
@@ -9,4 +11,4 @@ struct gfx_config {
     int monitor;
 };
 
-void gfx_hook_init(const struct gfx_config *cfg);
+void gfx_hook_init(const struct gfx_config *cfg, HINSTANCE self);