Linux: Fix PRIME detection on Steam

To avoid trying to do PRIME detection on fake `libGL.so` as used by e.g.
Renderdoc or Primus, we skip detection if there's a `libGL.so` in
`LD_LIBRARY_PATH`... and our luck is that Steam defines it and includes
system paths too, thus the actual system `libGL`... 🤦

So if we detect Steam, we skip this check.

Co-authored-by: Hein-Pieter van Braam-Stewart <[email protected]>

platform/linuxbsd/display_server_x11.cpp

@@ -4030,7 +4030,10 @@ DisplayServerX11::DisplayServerX11(const String &p_rendering_driver, WindowMode
 				use_prime = 0;
 			}
 
-			if (getenv("LD_LIBRARY_PATH")) {
+			// Some tools use fake libGL libraries and have them override the real one using
+			// LD_LIBRARY_PATH, so we skip them. *But* Steam also sets LD_LIBRARY_PATH for its
+			// runtime and includes system `/lib` and `/lib64`... so ignore Steam.
+			if (use_prime == -1 && getenv("LD_LIBRARY_PATH") && !getenv("STEAM_RUNTIME_LIBRARY_PATH")) {
 				String ld_library_path(getenv("LD_LIBRARY_PATH"));
 				Vector<String> libraries = ld_library_path.split(":");