Set u_AlphaThreshold after binding the shader program

glUniform* calls act on the currently bound shader, so setting the uniform before binding the shader is an error.
Setting the uniform before binding the shader will cause notable alpha testing issues because our uniform firewall would not match the uniform's actual value.
Fix COM_ParseExt to not skip top-bit-set characters where char is signed.
Make the (overly-simplistic) censorship code somewhat Unicode-aware.

Non-ASCII censored tokens need to be lower-case at present.
Clean up string comparisons.

Eliminate Q_strcasecmp, Q_strncasecmp and Q_stricmpn.
Ensure that Q_strncmp, Q_stricmp and Q_strnicmp handle NULL pointers.
Ensure that return values are correct (wrt strcmp & strcasecmp).

Case-insensitive comparison is not yet Unicode-aware.