Merge pull request #41 from mtobiasm/nowindow

RuntimeError "Cannot detect window"

_zengl.py

@@ -250,13 +250,17 @@ def __init__(self):
         if sys.platform.startswith("win"):
             lib = ctypes.WinDLL("Opengl32.dll")
             proc = ctypes.cast(lib.wglGetProcAddress, ctypes.CFUNCTYPE(ctypes.c_ulonglong, ctypes.c_char_p))
+            if not lib.wglGetCurrentContext():
+                raise RuntimeError('Cannot detect window with OpenGL support')
 
             def loader(name):
                 return proc(name.encode()) or ctypes.cast(lib[name], ctypes.c_void_p).value
 
         elif sys.platform.startswith("linux"):
             lib = ctypes.CDLL("libGL.so")
             proc = ctypes.cast(lib.glXGetProcAddress, ctypes.CFUNCTYPE(ctypes.c_ulonglong, ctypes.c_char_p))
+            if not lib.glxGetCurrentContext():
+                raise RuntimeError('Cannot detect window with OpenGL support')
 
             def loader(name):
                 return proc(name.encode()) or ctypes.cast(lib[name], ctypes.c_void_p).value