cvar: r_texture_dds_load_dxt1_noalpha; if set, DXT1 alpha detection is disabled,...

cvar: r_texture_dds_load_dxt1_noalpha; if set, DXT1 alpha detection is disabled, and DXT1 is assumed to have no alpha.
Rationale is that ATI Compressonator sometimes picks the alpha'd compression mode [a, (a+b)/2, b, transparent] and then never uses the transparent color value 3, as it sometimes can yield better results than the non-alpha'd compression mode [a, (2a+b)/3, (a+2b)/3, b], and this throws off alpha detection on loading

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@10375 d7cf8633-e32d-0410-b094-e92efae38249