Declare interface types as void and remove cast macros

This declares the interface types CoglFramebuffer, CoglBuffer,
CoglTexture, CoglMetaTexture and CoglPrimitiveTexture as void when
including the public cogl.h header so that users don't have to use lots
of C type casts between instance types and interface types.

This also removes all of the COGL_XYZ() type cast macros since they do
nothing more than compile time type casting but it's less readable if
you haven't seen that coding pattern before.

Unlike with gobject based apis that use per-type macros for casting and
performing runtime type checking we instead prefer to do our runtime
type checking internally within the front-end public apis when objects
are passed into Cogl. This greatly reduces the verbosity for users of
the api and may help reduce the chance of excessive runtime type
checking that can sometimes be a problem.

(cherry picked from commit 248a76f5eac7e5ae4fb45208577f9a55360812a7)

Since we can't break the 1.x api this version of the patch actually
defines compatible NOP macros within deprecated/cogl-type-casts.h