cogl-texture: List texture subclass types rather than hardcoding them

Instead of having a hardcoded series of if-statements in
cogl_is_texture to determine which types should appear as texture
subclasses, they are now stored in a GSList attached to the Cogl
context. The list is amended to using a new cogl_texture_register_type
function. There is a convenience macro called COGL_TEXTURE_DEFINE
which uses COGL_HANDLE_DEFINE_WITH_CODE to register the texture type
when the _get_type() function is first called.

cogl/cogl-context.h

@@ -162,6 +162,10 @@ typedef struct
   CoglMaterialProgramType current_use_program_type;
   GLuint                  current_gl_program;
 
+  /* List of types that will be considered a subclass of CoglTexture in
+     cogl_is_texture */
+  GSList           *texture_types;
+
   CoglContextDriver drv;
 } CoglContext;