Enable libGL workaround on any system where it's available

Fixes #3772

doc/changelog.asciidoc

@@ -79,6 +79,8 @@ Fixed
 - Window ID 0 is now excluded properly from `:tab-take` completion.
 - A rare crash when cancelling a download has been fixed.
 - The Makefile (intended for packagers) now supports `PREFIX` properly.
+- The workaround for a black window with Nvidia graphics is now enabled on
+  non-Linux systems (like FreeBSD) as well.
 
 v1.2.1
 ------

qutebrowser/misc/backendproblem.py

@@ -166,8 +166,9 @@ def _nvidia_shader_workaround():
     See https://bugs.launchpad.net/ubuntu/+source/python-qt4/+bug/941826
     """
     assert objects.backend == usertypes.Backend.QtWebEngine, objects.backend
-    if utils.is_linux:
-        ctypes.CDLL(ctypes.util.find_library("GL"), mode=ctypes.RTLD_GLOBAL)
+    libgl = ctypes.util.find_library("GL")
+    if libgl is not None:
+        ctypes.CDLL(libgl, mode=ctypes.RTLD_GLOBAL)
 
 
 def _handle_nouveau_graphics():