core: Make the form loader use Windows-1252 if SWF version <= 5

The form loader now loads files using Windows-1252 if the SWF version is
smaller than 6. This roughly matches Flash's behaviour (Flash uses
Windows-1252 on Windows, on macOS a slightly different custom encoding
is used).
Previously, UTF-8 has been (wrongly) used for all SWF files if
System#useCodepage hasn't been set to true, leading to incorrectly
displayed characters.

core/src/avm1/globals/system.rs

@@ -265,8 +265,8 @@ pub struct SystemProperties {
     /// If true then settings should be saved and read from the exact same domain of the player
     /// If false then they should be saved to the super domain
     pub exact_settings: bool,
-    /// If true then the system codepage should be used instead of unicode for text files
-    /// If false then unicode should be used
+    /// If true, the system codepage should be used for text files
+    /// If false, UTF-8 should be used for SWF version >= 6 and ISO Latin-1 for SWF version <= 5
     pub use_codepage: bool,
     /// The capabilities of the player
     pub capabilities: SystemCapabilities,

core/src/loader.rs

@@ -31,7 +31,7 @@ use crate::tag_utils::SwfMovie;
 use crate::vminterface::Instantiator;
 use crate::{avm2_stub_method, avm2_stub_method_context};
 use chardetng::EncodingDetector;
-use encoding_rs::UTF_8;
+use encoding_rs::{UTF_8, WINDOWS_1252};
 use gc_arena::{Collect, GcCell};
 use indexmap::IndexMap;
 use ruffle_render::utils::{determine_jpeg_tag_format, JpegTagFormat};
@@ -1257,6 +1257,9 @@ impl<'gc> Loader<'gc> {
                     // Convert the text into UTF-8
                     utf8_string = encoding.decode(&body).0;
                     utf8_string.as_bytes()
+                } else if activation.context.swf.version() <= 5 {
+                    utf8_string = WINDOWS_1252.decode(&body).0;
+                    utf8_string.as_bytes()
                 } else {
                     &body
                 };