Set multiview as the default for stereoscopic rendering (#9682)

Enables the multiview implementation as the default for stereoscopic
rendering. Now all STE variants use the multiview path.

This change removes all CMake configurations, build scripts, and C++
preprocessors previously used for selecting stereoscopic rendering
modes. And, all shaders are now compiled for multiview.

The instanced rendering implementation is going to be removed. Note that
this commit only handles switching the default. The actual removal of
instanced rendering code will be submitted as a separate follow-up
commit.

BUGS=[470198472]

samples/hellostereo.cpp

@@ -160,12 +160,6 @@ static int handleCommandLineArguments(int argc, char* argv[], App* app) {
 }
 
 int main(int argc, char** argv) {
-
-#if !defined(FILAMENT_SAMPLES_STEREO_TYPE_MULTIVIEW)
-    std::cerr << "This sample only works with multiview enabled.\n";
-    exit(1);
-#endif
-
     App app{};
     app.config.title = "stereoscopic rendering";
     handleCommandLineArguments(argc, argv, &app);