Merge OpenGL API changes and better modern OpenGL support

Closes https://github.com/wxWidgets/wxWidgets/pull/227

docs/changes.txt

@@ -59,6 +59,8 @@ Changes in behaviour which may result in build errors
 
 - Added wxNativeWindow allowing to embed native widgets in wxWidgets programs.
 
+- Improve wxGLCanvas to be more type safe and better support modern OpenGL.
+
 All:
 
 - Add UTF-8 and ZIP 64 support to wxZip{Input,Output}Stream (Tobias Taschner).

docs/doxygen/mainpages/samples.h

@@ -553,6 +553,8 @@ commands through the menu.
     Arrow keys rotate the cube. Space bar toggles spinning.
 @li @b isosurf Draws a surface by reading coordinates from a DAT file.
 @li @b penguin Draws a rotatable penguin by reading data from a DXF file.
+@li @b pyramid Draws a rotatable tetrahedron and some strings.
+    It uses OpenGL 3.2 Core Profile context.
 
 @sampledir{opengl}