Fix #12197: Force set /utf-8 for msvc compilers (#12198)

The MSVC compiler determines the encoding of the source code based on
the BOM of the source code when reading it. If there is no BOM, it
defaults to the local encoding, which is gb2312, codepage 936, on
Simplified Chinese Windows. This can cause errors such as newline
characters in strings.

CMakeLists.txt

@@ -13,6 +13,12 @@ else()
   set(SDL3_MAINPROJECT OFF)
 endif()
 
+# Add UTF-8 encoding support for MSVC compiler.
+# This ensures that the MSVC compiler interprets source files as UTF-8 encoded,
+# which is useful for projects containing non-ASCII characters in source files.
+add_compile_options("$<$<C_COMPILER_ID:MSVC>:/utf-8>")
+add_compile_options("$<$<CXX_COMPILER_ID:MSVC>:/utf-8>")
+
 # By default, configure SDL3 in RelWithDebInfo configuration
 if(SDL3_MAINPROJECT)
   get_property(is_multi_config GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)