Don't include core/io/image.h in core/os/os.h

`core/os/os.h` doesn't use `core/io/image.h`. It just brings
transitive dependencies. Lots of dependencies because `core/os/os.h`
is transitively included in almost every file of godot

Also added `core/io/image.h` into files^1 where `Ref<Image>` and `core/os/os.h`
were used to prevent obscure errors involving `Ref<Image>`

^1 except those which include `core/io/image_loader.h` or `core/io/image.h` by
corresponding .h file with the same name

Signed-off-by: Yevhen Babiichuk (DustDFG) <dfgdust@gmail.com>
Co-authored-by: A Thousand Ships <96648715+AThousandShips@users.noreply.github.com>

modules/tinyexr/image_saver_tinyexr.cpp

@@ -31,6 +31,7 @@
 #include "image_saver_tinyexr.h"
 
 #include "core/math/math_funcs.h"
+#include "core/os/os.h"
 
 #include <zlib.h> // Should come before including tinyexr.
 

modules/tinyexr/image_saver_tinyexr.h

@@ -31,7 +31,7 @@
 #ifndef IMAGE_SAVER_TINYEXR_H
 #define IMAGE_SAVER_TINYEXR_H
 
-#include "core/os/os.h"
+#include "core/io/image.h"
 
 Error save_exr(const String &p_path, const Ref<Image> &p_img, bool p_grayscale);
 Vector<uint8_t> save_exr_buffer(const Ref<Image> &p_img, bool p_grayscale);