fix crash when removing synth from midi-track

(and there are bypassed audio-plugins after said synth)
fix routing display for mono synths and midi-tracks w/o synth
fix #5840 ; redefinition of typedef 'VstTimeInfo'
comment unused function argument
replace standards-wobbling variable-length-arrays with alloca()
add new folders to linux vst search path to help out AVLinux and some other specialized distros
prevent a strange crash while iterating over tracks (now, and maybe in the future)
fix up the abomination caused by moving from variable length (multidimensional) arrays to alloca'ed arrays, specifically access to arr[a][b].

This needs checking by an actual VBAP+multispeaker user.
fix thinko. custom panner URL is remembered for later when port-count/panner changes
fix vbap-speaker calculations to work with alloca()
move check for n_speakers so that we avoid needless work
Merge branch 'master' into cairocanvas