Added emblem blank pixels check

- Made possible thanks to (and using base code by) Ai4rei - eAthena
  r15263 and add-on patch
  http://hercules.ws/board/topic/2974-add-on-patch-for-ea-r15263-commit/
- Modified to allow checking for a percentage of transparent pixels
  rather than full image transparency.
- Tweaked to accept some other bmp formats that are currently allowed
  by the client.
- The performance hit from enabling this check is negligible (benchmarks
  show that scanning an emblem requires about half the time required to
  decompress it after it's received), but it does exist, as noted in the
  configuration file.
- Special thanks to Ind.

Signed-off-by: Haru <haru@dotalux.com>