Rename main.c to sdl2-sdlrenderer.c and implement sdl2-opengl3 example

compile

@@ -3,7 +3,10 @@
 CC=clang
 CFLAGS="-g -Wall $(pkg-config --cflags sdl2) -Wl,-rpath,\$ORIGIN/lib -Ivendor"
 LIBS="$(pkg-config --libs sdl2) -Llib -lcimgui"
-SRC=src/*.c
-OUT=main
+SDL2_SDLRENDERER=src/sdl2-sdlrenderer.c
+SDLRENDERER_OUT=sdl2-sdlrenderer
+SDL2_OPENGL3=src/sdl2-opengl3.c
+OPENGL3_OUT=sdl2-opengl3
 
-(set -x ; $CC $CFLAGS $LIBS $SRC -o $OUT)
+(set -x ; $CC $CFLAGS $LIBS $SDL2_SDLRENDERER -o $SDLRENDERER_OUT)
+(set -x ; $CC $CFLAGS $LIBS -lGL -ldl $SDL2_OPENGL3 -o $OPENGL3_OUT)