Make GL version setting specific to Mac OS only

Refining earlier change (kyamagu#214). It turns out that Xvfb's GL behavior on Linux
differ depending on whether there is actual graphic hardware or entirely
headless. The Apple GL settings works on Xvfb on my computer
(AMD Radeon R5 Graphics), but fails in github headless CI.

Fixes kyamagu#266

docs/tutorial/canvas.rst

@@ -91,10 +91,11 @@ The following example uses glfw package to create an OpenGL context. Install
         glfw.window_hint(glfw.VISIBLE, glfw.FALSE)
         glfw.window_hint(glfw.STENCIL_BITS, 8)
         # see https://www.glfw.org/faq#macos
-        glfw.window_hint(glfw.CONTEXT_VERSION_MAJOR, 3)
-        glfw.window_hint(glfw.CONTEXT_VERSION_MINOR, 2)
-        glfw.window_hint(glfw.OPENGL_FORWARD_COMPAT, True)
-        glfw.window_hint(glfw.OPENGL_PROFILE, glfw.OPENGL_CORE_PROFILE)
+        if sys.platform.startswith("darwin"):
+            glfw.window_hint(glfw.CONTEXT_VERSION_MAJOR, 3)
+            glfw.window_hint(glfw.CONTEXT_VERSION_MINOR, 2)
+            glfw.window_hint(glfw.OPENGL_FORWARD_COMPAT, True)
+            glfw.window_hint(glfw.OPENGL_PROFILE, glfw.OPENGL_CORE_PROFILE)
         window = glfw.create_window(640, 480, '', None, None)
         glfw.make_context_current(window)
         yield window
@@ -144,10 +145,11 @@ Here's a complete example:
             raise RuntimeError('glfw.init() failed')
         glfw.window_hint(glfw.STENCIL_BITS, 8)
         # see https://www.glfw.org/faq#macos
-        glfw.window_hint(glfw.CONTEXT_VERSION_MAJOR, 3)
-        glfw.window_hint(glfw.CONTEXT_VERSION_MINOR, 2)
-        glfw.window_hint(glfw.OPENGL_FORWARD_COMPAT, True)
-        glfw.window_hint(glfw.OPENGL_PROFILE, glfw.OPENGL_CORE_PROFILE)
+        if sys.platform.startswith("darwin"):
+            glfw.window_hint(glfw.CONTEXT_VERSION_MAJOR, 3)
+            glfw.window_hint(glfw.CONTEXT_VERSION_MINOR, 2)
+            glfw.window_hint(glfw.OPENGL_FORWARD_COMPAT, True)
+            glfw.window_hint(glfw.OPENGL_PROFILE, glfw.OPENGL_CORE_PROFILE)
         window = glfw.create_window(WIDTH, HEIGHT, '', None, None)
         glfw.make_context_current(window)
         yield window

tests/conftest.py

@@ -16,10 +16,11 @@ def glfw_context():
     glfw.window_hint(glfw.VISIBLE, glfw.FALSE)
     glfw.window_hint(glfw.STENCIL_BITS, 8)
     # see https://www.glfw.org/faq#macos
-    glfw.window_hint(glfw.CONTEXT_VERSION_MAJOR, 3)
-    glfw.window_hint(glfw.CONTEXT_VERSION_MINOR, 2)
-    glfw.window_hint(glfw.OPENGL_FORWARD_COMPAT, True)
-    glfw.window_hint(glfw.OPENGL_PROFILE, glfw.OPENGL_CORE_PROFILE)
+    if sys.platform.startswith("darwin"):
+        glfw.window_hint(glfw.CONTEXT_VERSION_MAJOR, 3)
+        glfw.window_hint(glfw.CONTEXT_VERSION_MINOR, 2)
+        glfw.window_hint(glfw.OPENGL_FORWARD_COMPAT, True)
+        glfw.window_hint(glfw.OPENGL_PROFILE, glfw.OPENGL_CORE_PROFILE)
     context = glfw.create_window(640, 480, '', None, None)
     glfw.make_context_current(context)
     logger.debug('glfw context created')