build: include <cogl/cogl-sdl.h>

cogl-sdl.h was recently split out into a top-level Cogl header because
automatically including SDL.h for all applications could cause problems
due to how SDL.h warns when __STRICT_ANSI__ isn't defined.
build: include stdlib.h/math.h where necessary

fixes some build warnings
build: Update to build with Cogl master

This makes sure Rig builds with Cogl master as of commit 55b09f8a