dpshaderkill enhancements

dpshaderkill enhancements

"Kill" current shader:
dpshaderkillifcvar r_foo == 42
dpshaderkillifcvar r_foo != 42
dpshaderkillifcvar r_foo < 42
dpshaderkillifcvar r_foo > 42
dpshaderkillifcvar r_foo <= 42
dpshaderkillifcvar r_foo >= 42

"Un-kill" current shader (unsets the kill flag):
dpnoshaderkillifcvar r_foo == 42
dpnoshaderkillifcvar r_foo != 42
dpnoshaderkillifcvar r_foo < 42
dpnoshaderkillifcvar r_foo > 42
dpnoshaderkillifcvar r_foo <= 42
dpnoshaderkillifcvar r_foo >= 42

Also, shader override logic is modified so killed shaders can be used to make
alternatives, e.g. switching between two shader definitions based on cvars:
- killed shaders never override other shaders, but show no warning
- non-killed shaders override killed shaders
- non-killed shaders still do not override other non-killed shaders

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@11287 d7cf8633-e32d-0410-b094-e92efae38249
::stable-branch::merge=4434beb3c023ef9f97f772f3944b5800065a8e15