Modify mj_PLUGIN_LIB_INIT to support multiple plugins in the same compilation unit.

Previously on MSVC we used DllMain to register plugins, you cannot have multiple definitions of DllMain in a single unit so you would get errors if you tried to register two plugins. This modifies the implementation to insert a function pointer into the C runtime initialization instead.

PiperOrigin-RevId: 896612678
Change-Id: I07732147b955d741c836acff6da986db0e9b9eff

src/experimental/mjz/mjz_decoder.cc

@@ -192,7 +192,7 @@ static mjSpec* ParseZipBuffer(const void* buffer, int nbuffer, const char* name,
   return mj_parseXML(root.c_str(), vfs, error, error_sz);
 }
 
-mjPLUGIN_LIB_INIT {
+mjPLUGIN_LIB_INIT(mjz_decoder) {
   mjpDecoder decoder;
   decoder.content_type = "application/zip";
   decoder.extension = ".mjz|.zip";

src/experimental/platform/hal/renderer.cc

@@ -238,7 +238,7 @@ void Renderer::UpdateFps() {
 
 }  // namespace mujoco::platform
 
-mjPLUGIN_LIB_INIT {
+mjPLUGIN_LIB_INIT(renderer) {
   mujoco::platform::GuiPlugin plugin;
   plugin.name = "Filament";
   plugin.update = [](mujoco::platform::GuiPlugin* self) {

src/experimental/platform/ux/object_launcher_plugin.cc

@@ -175,7 +175,7 @@ class ObjectLauncher {
 
 }  // namespace mujoco::studio
 
-mjPLUGIN_LIB_INIT {
+mjPLUGIN_LIB_INIT(object_launcher) {
   using mujoco::studio::ObjectLauncher;
 
   static ObjectLauncher plugin;