Court textures (grass, clay, hard) and clean-up

Court textures (grass, clay, hard) and clean-up

Re-work court graphic to support different court textures
and convert grass texture from old court + add new textures:
 - Clay (red/orange-ish) and
 - Hard (concrete-looking grey)

Don't call SDL_SetColorKey() for loaded images anymore, because
we're now using RGBA PNG files instead of "black-is-alpha" BMPs.

Removed introimage() function as it's not used anywhere.