Change default value of r_shadows_shadowmapscale from 1 to 0.25, so that

Change default value of r_shadows_shadowmapscale from 1 to 0.25, so that
shadows cover a larger part of the level around the eye.

Change the way r_shadows 2 size is calculated so that it uses 1/4th of
the texture atlas size (at default this would be 2048 of 8192) rather
than 2*r_shadow_shadowmapping_maxsize (at default that was 2*512 = 1024
of 8192).

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12375 d7cf8633-e32d-0410-b094-e92efae38249