[Mechanics] Effects slowing down cast speed also affect GCD

[Jerrythemagicdog]

Description:

Effects like Mind-Numbing Poison (Item ID: 9186) also slow down Global Cooldown which they shouldn't.
(They already can't reduce the gcd above 1.5 and that is working)

Expected behaviour:

Global Cooldowns should be unaffected by spells like Slow, Mind-Numbing poison effect and Curse of Tongues. It's a known bug across many private servers.

** Reproduce steps:**

• Cast Icy Veins on mage
• Check GCD for an instant spell
• Apply mind numbing on mage
• Check GCD for an instant spell, and see that it has increased

Some related issues and conclusions:
ccshiro/cc-buglist#248
(kelno: this bug report is actually about getting above 1.5 and not about this exact problem)

rev. hash/commit:
rev. 10603

[kelno]

(from chat)
-- Kelno
We never send the gcd to the client, what we do is
We update the player field UNIT_MOD_CAST_SPEED whenever a buff/debuff is applied
Which is a multiplier, 1 by default
If you get icy veins it gets down to 0.8333 for example
And what this does affect that I can see so far is:

• The GCD
• The shown cast time on tooltips

So I find this very strange
Because changing this would also break the cast time on tooltips
-- Jerry
so two things are controlled by one thing
maybe GCD shouldn't be in UNIT_MOD_CAST_SPEED at all
-- Kelno
Yes, maybe there are more than two but whatever
Ah well that's not something we can control
That's just how the client decides the GCD
-- Jerry
okay, so how did Warmane do it
hmmm
-- Kelno
Maybe they broke the tooltip in that case

[kelno]

Apparently it's not the case on warmane either (tested by jerry):
https://streamable.com/ami5k

https://docs.google.com/document/d/17EZjEeuVQ_oziqph88E_in1WlmTmA5n1ZcJuEbdCr7Q/edit?usp=sharing

[kelno]

Conclusion here is: It should be affected.