proton: Look for libraries in Steam Client path.

This is used to pick up Valve's build of ffmpeg, which includes proprietary decoders.

Signed-off-by: Derek Lesho <dlesho@codeweavers.com>

proton

@@ -891,6 +891,9 @@ class CompatData:
 
             setup_game_dir_drive()
 
+            # add Steam ffmpeg libraries to path
+            prepend_to_env_str(g_session.env, ld_path_var, steamdir + "/ubuntu12_64/video/:" + steamdir + "/ubuntu12_32/video/", ":")
+
 def comma_escaped(s):
     escaped = False
     idx = -1