cogl-buffer: Handle subclass registration like cogl-texture

Instead of having to extend cogl_is_buffer with new buffer types
manually this now adds a new COGL_BUFFER_DEFINE macro to be used instead
of COGL_OBJECT_DEFINE for CoglBuffer subclasses. This macro will
automatically register the new type with ctx->buffer_types which will
iterated by cogl_is_buffer. This is the same coding pattern used for
CoglTexture.

cogl/cogl-pixel-buffer.c

@@ -84,7 +84,7 @@ cogl_pixel_buffer_vtable;
 static const CoglBufferVtable
 cogl_malloc_pixel_buffer_vtable;
 
-COGL_OBJECT_DEFINE (PixelBuffer, pixel_buffer)
+COGL_BUFFER_DEFINE (PixelBuffer, pixel_buffer)
 
 CoglPixelBuffer *
 cogl_pixel_buffer_new (unsigned int size)