Support reversed Z rendering and float32 offscreen depth buffer

Combined, both options greatly improve camera depth accuracy compared to
OpenGL default Z mapping and int24 depth buffer.

Requires GL_ARB_clip_control and ARB_depth_buffer_float extensions

simulate/platform_ui_adapter.cc

@@ -27,6 +27,7 @@ void PlatformUIAdapter::FreeMjrContext() {
 
 bool PlatformUIAdapter::RefreshMjrContext(const mjModel* m, int fontscale) {
   if (m != last_model_ || fontscale != last_fontscale_) {
+    // con_.depthMapping =   mjDB_ONETOZERO;
     mjr_makeContext(m, &con_, fontscale);
     last_model_ = m;
     last_fontscale_ = fontscale;