Add Stages parameter to Phaser

[mkruselj]

Presently, the phaser in Surge has 4 stages per channel (so 8 total). I could totally see the use for Stages parameter, that would be an integer between 2 and 12, to change the number of allpass filters used by the effect. Of course that means 4-24 stages total, for full stereo.

[baconpaul]

This is still a cool idea but I’m gonna knock it out if 17 beta 1

[mkruselj]

Thanks to @amyfurniss this is now a reality in the nightlies! It needs a bit more work to sound better (introducing legacy mode that keeps old 4 bands setup, but in new modes it should use the spread/distribution parameters to calculate the frequencies and modulation scaling per allpass.

[mkruselj]

With #3015 this issue is now finally put to rest. Thank you @amyfurniss for being awesome!