Comments to redirect people to OpenGL3 examples (#1116)

Fixed overzealous GCC warnings (#883) + partly undo 8553bab

Minor const fixes for overzealous warnings (#883)

Added a void* user_data parameter to Clipboard function handlers. (#875)

Examples: OpenGL2: Uploading font texture as RGBA32 to increase compatibility with users shaders for beginners (#824)

Examples: OpenGL*: Savnig/restoring existing scissor rectangle for completeness (#807)

Examples: Accessing ImVector fields directly, feel less stl-ey and fix renderer side assert when render list only contains a callback (#810)

Examples: Renaming opengl_example/ to opengl2_example/ for clarity (1/2 because git)