Add begin_validate() and end_validate() virtual functions, and
corresponding free functions, to GimpTileHandlerValidate.  These
functions are called before/after validation happens, and should
perform any necessary steps to prepare for validation.  The default
implementation suspends validation on tile access, so that the
assigned buffer may be accessed without causing validation.

Implement the new functions in GimpTileHandlerProjectable, by
calling gimp_projectable_begin_render() and
gimp_projectable_end_render(), respectively, instead of calling
these functions in the ::validate() implementation (which, in turn,
allows us to use the default ::validate() implementation.)

In GimpProjection, use the new functions in place of
gimp_projectable_{begin,end}_render().