gemrb: Update to 0.8.7

GemRB v0.8.7 (2020-08-23):
  New features:
    - new smarter pathfinder with bumping support
    - animal taming, iwd2 hardcoded saving throw bonuses
    - vcpkg and out-of-the box msvc support
    - non-ascii data filename support

  Improved features:
    - disk reading speedups, ease of setup
    - iwd chargen, pst spell timing, hardcoded overlays, iwd2 casting ai
    - better actor speeds & walk sounds
    - audio, pst ini handling, savegame compatibility, morale handling
    - effects, projectiles, actions, range calculations
    - bugfixes

  Notes:
    - switched to c++11 and cmake 3.1 as a minimum
    - dropped windows 9x compatibility

(nia)