Examples: SDL/OpenGL: Tabs to spaces

Examples: SDL2+OpenGL3: Fix context creation options (#468, #463)

Examples: Header comments.

Examples: Clarified comments. Removed font merging example from comments.

Examples: SDL+OpenGL3: Various fixes to example to make it matches current GL3 example (#356)

Examples: SDL+OpenGL3: Using glew like existing example + renaming (#356)

Previous SDL implementation didn't work with my OpenGL 3.3 project, spliced the GLFW OpenGL examples on top of the SDL example to create an option that works for me. Could be useful for others.