Improve new wxGLCanvas compatibility with old hardware and code

Combined patch with the following changes:
- Don't add wxGLAttributes::Defaults() when the attributes-list is NULL.
- Add display default attributes used in wx versions before 3.1 when the
  attributes-list is NULL. These attributes are different for each platform.
- Fix wxMSW PixelFormatDescriptor initialization.
- Don't set color buffers when RGBA() is used.
- Fix setting colour sizes in OS X and a few other fixes.
- Make documentation more clear about these subjects.

Closes #17425.

include/wx/glcanvas.h

@@ -161,6 +161,11 @@ public:
     wxGLAttributes& Samplers(int val);
     wxGLAttributes& FrameBuffersRGB();
     void EndList(); // No more values can be chained
+
+    // This function is undocumented and can not be chained on purpose!
+    // To keep backwards compatibility with versions before wx3.1 we add here
+    // the default values used in those versions for the case of NULL list.
+    void AddDefaultsForWXBefore31();
 };
 
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