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.

src/osx/glcanvas_osx.cpp

@@ -180,17 +180,17 @@ void wxGLContextAttrs::EndList()
 
 wxGLAttributes& wxGLAttributes::RGBA()
 {
-    AddAttribute(WXOpenGLPFAColorSize);
-    AddAttribute(24);
-    AddAttribute(WXOpenGLPFAAlphaSize);
-    AddAttribute(8);
+    // No effect
     return *this;
 }
 
 wxGLAttributes& wxGLAttributes::BufferSize(int val)
 {
-    // No effect
-    wxUnusedVar(val);
+    if ( val >= 0 )
+    {
+        AddAttribute(WXOpenGLPFAColorSize);
+        AddAttribute(val);
+    }
     return *this;
 }
 
@@ -225,6 +225,10 @@ wxGLAttributes& wxGLAttributes::AuxBuffers(int val)
 
 wxGLAttributes& wxGLAttributes::MinRGBA(int mRed, int mGreen, int mBlue, int mAlpha)
 {
+    // NOTE: 'NSOpenGLPixelFormat' doesn't offer a value for each of the RGBA
+    //   components, but just one for the size of the colour buffer. In order
+    //   to make wx more effective, this function _does_ set the colour buffer.
+
     int minColorBits = 0;
     if ( mRed > minColorBits )
         minColorBits = mRed;
@@ -232,6 +236,9 @@ wxGLAttributes& wxGLAttributes::MinRGBA(int mRed, int mGreen, int mBlue, int mAl
         minColorBits = mGreen;
     if ( mBlue > minColorBits )
         minColorBits = mBlue;
+    // Now that we have the R/G/B maximum, obtain a minimun for the buffer.
+    minColorBits *= 4; // 'alpha' included
+
     if ( minColorBits > 0 )
     {
         AddAttribute(WXOpenGLPFAColorSize);
@@ -268,6 +275,7 @@ wxGLAttributes& wxGLAttributes::Stencil(int val)
 
 wxGLAttributes& wxGLAttributes::MinAcumRGBA(int mRed, int mGreen, int mBlue, int mAlpha)
 {
+    // See MinRGBA() for some explanation.
     int minAcumBits = 0;
     if ( mRed > minAcumBits )
         minAcumBits = mRed;
@@ -275,15 +283,14 @@ wxGLAttributes& wxGLAttributes::MinAcumRGBA(int mRed, int mGreen, int mBlue, int
         minAcumBits = mGreen;
     if ( mBlue > minAcumBits )
         minAcumBits = mBlue;
-    if ( minAcumBits > 0 )
+    if ( mAlpha > minAcumBits )
+        minAcumBits = mAlpha;
+    minAcumBits *= 4;
+    if ( minAcumBits >= 0 )
     {
         AddAttribute(WXOpenGLPFAAccumSize);
         AddAttribute(minAcumBits);
     }
-
-    // No effect for Alpha in accumulation buffer
-    wxUnusedVar(mAlpha);
-
     return *this;
 }
 
@@ -352,6 +359,15 @@ wxGLAttributes& wxGLAttributes::Defaults()
     return *this;
 }
 
+void wxGLAttributes::AddDefaultsForWXBefore31()
+{
+    // ParseAttribList() will add EndList(), don't do it now
+    DoubleBuffer();
+    // Negative value will keep its buffer untouched
+    BufferSize(8).Depth(8).MinRGBA(-1, -1, -1, 0);
+}
+
+
 // ----------------------------------------------------------------------------
 // wxGLContext
 // ----------------------------------------------------------------------------