Include hook_imagecache_preprocess_cache

Throughout various projects I've had to add specific validation before actually building the image cache itself. I'd like to suggest the inclusion of a hook (imagecache_preprocess_cache) just before building the cache inside _imagecache_cache (imagecache.module):

// Preprocess image before cache is generated.
  module_invoke_all('imagecache_preprocess_cache', $preset, $src, $dst);

  // check if deriv exists... (file was created between apaches request handler and reaching this code)
  // otherwise try to create the derivative.
  if (file_exists($dst) || imagecache_build_derivative($preset['actions']....

This will allow other modules to intercept the imagecache processing before the image starts being processed.