mangohud support: getLibraryString should return absolute path when possible

Some distros ship MangoHub vulkan layer json with bare shared object name
instead of an absolute path. This breaks environment config as
`MinecraftInstance::createLaunchEnvironment()` seems to require absolute
path of `libMangoHud.so`.

Since we already have `MangoHud::findLibrary()` lying around, use that to
figure out where `libMangoHud.so` really is. In case of a platform that
doesn't support `dlopen()`, fallback to old behavior and return the path
verbatim as it is recorded in vk layer json.

Signed-off-by: Tianhao Chai <cth451@gmail.com>

launcher/minecraft/MinecraftInstance.cpp

@@ -608,7 +608,7 @@ QProcessEnvironment MinecraftInstance::createLaunchEnvironment()
             // dlsym variant is only needed for OpenGL and not included in the vulkan layer
             appendLib("libMangoHud_dlsym.so");
             appendLib("libMangoHud_opengl.so");
-            appendLib(mangoHudLib.fileName());
+            preloadList << mangoHudLibString;
         }
 
         env.insert("LD_PRELOAD", preloadList.join(QLatin1String(":")));