Add texture function, texture color lookup

Add Color::SetRGB888 member function (set float vector color from
integer RGB values).

Rename TEXTURE_TYPE_USE_RAW_TEXTURE to
TEXTURE_TYPE_USE_UNCOMPRESSED_TEXTURE.

Add sreTexture::TextureLookupNearest function (simulate GPU texture
lookup).

Reduce terrain scale for demo4.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>