build: Make cogl.h not conditional on driver defines

build: Make cogl.h not conditional on driver defines

The main COGL header cogl.h is currently created at configure time
because it conditionally includes the driver-dependent defines. This
sometimes leads to a stale cogl.h with old definitions which can
break the build until you clean out the whole tree and start from
scratch.

We can generate a stable cogl-defines.h at build time from the
equivalent driver-dependent header and let cogl.h include that
file instead.