Make CoglTexture2D public as experimental API

This exposes a CoglTexture2D typedef and adds the following experimental
API:
    cogl_is_texture_2d
    cogl_texture_2d_new_with_size
    cogl_texture_2d_new_from_data
    cogl_texture_2d_new_from_foreign

Since this is experimental API you need to define
COGL_ENABLE_EXPERIMENTAL_API before including cogl.h.

Note: With these new entrypoints we now expect a CoglContext pointer to
be passed in, instead of assuming there is a default context. The aim is
that for Cogl 2.0 we won't have a default context so this is a step in
that direction.

cogl/cogl-blit.c

@@ -199,7 +199,7 @@ _cogl_blit_copy_tex_sub_image_begin (CoglBlitData *data)
   _COGL_GET_CONTEXT (ctx, FALSE);
 
   /* This will only work if the target texture is a CoglTexture2D */
-  if (!_cogl_is_texture_2d (data->dst_tex))
+  if (!cogl_is_texture_2d (data->dst_tex))
     return FALSE;
 
   fbo = _cogl_offscreen_new_to_texture_full