update SDL_mixer to SDL3_mixer 3.2.0

libs/SDL3_mixer/docs/INTRO-emscripten.md

@@ -0,0 +1,52 @@
+
+# Introduction to SDL_mixer with Emscripten
+
+The easiest way to use SDL_mixer is to include it along with SDL as subprojects in your project.
+
+We'll start by creating a simple project to build and run [hello.c](hello.c)
+
+First, you should have the Emscripten SDK installed from:
+
+https://emscripten.org/docs/getting_started/downloads.html
+
+Create the file CMakeLists.txt
+```cmake
+cmake_minimum_required(VERSION 3.16)
+project(hello)
+
+# set the output directory for built objects.
+# This makes sure that the dynamic library goes into the build directory automatically.
+set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/$<CONFIGURATION>")
+set(CMAKE_LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/$<CONFIGURATION>")
+
+# This assumes the SDL source is available in vendored/SDL
+add_subdirectory(vendored/SDL EXCLUDE_FROM_ALL)
+
+# This assumes the SDL_mixer source is available in vendored/SDL_mixer
+add_subdirectory(vendored/SDL_mixer EXCLUDE_FROM_ALL)
+
+# on Web targets, we need CMake to generate a HTML webpage.
+if(EMSCRIPTEN)
+  set(CMAKE_EXECUTABLE_SUFFIX ".html" CACHE INTERNAL "")
+endif()
+
+# Create your game executable target as usual
+add_executable(hello WIN32 hello.c)
+
+# Link to the actual SDL3 library.
+target_link_libraries(hello PRIVATE SDL3_mixer::SDL3_mixer SDL3::SDL3)
+```
+
+Build:
+```sh
+emcmake cmake -S . -B build
+cd build
+emmake make
+```
+
+You can now run your app by pointing a webserver at your build directory and connecting a web browser to it, opening hello.html
+
+A more complete example is available at:
+
+https://github.com/Ravbug/sdl3-sample
+