Revert most of "Examples: refactor all examples with a MainLoopStep() function..." (#2492, #3699)"

This reverts commit 96ab68eee079620b9a2d0e12e9978ade9ad32711.

# Conflicts:
#	docs/CHANGELOG.txt
#	examples/example_glfw_opengl3/main.cpp
#	examples/example_sdl_opengl3/main.cpp