DDS with DXT1a: was not correctly detected if r_texture_dds_load_alphamod == 0 (it...

DDS with DXT1a: was not correctly detected if r_texture_dds_load_alphamod == 0 (it does not really check for DDPF_ALPHAPIXELS), fixing.
ODE: physics_ode_constantstep now can be fractional, in this case ODE will run at this tickrate instead of using sys_ticrate, so ODE run at different framerate than server (may be useful for high values, such like running ODE at 100 fps and server at 40fps).

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@11541 d7cf8633-e32d-0410-b094-e92efae38249