pc: add option to translate absolute analog values to fader input (#375)

## Link to GitHub Issue, if one exists
n/a

## Description of change
Add a new option that allows absolute position analog input, such as
SDVX controller knobs and IIDX turntables, to be bound as input for
Polaris Chord faders.

In practice this means turning a knob counter-clockwise results in fader
being held left (relative to velocity), and vice versa.

## Testing
Tested using FauceTwo and a mouse.

src/spice2x/games/pc/pc.h

@@ -5,6 +5,7 @@
 namespace games::pc {
     extern std::string PC_INJECT_ARGS;
     extern bool PC_NO_IO;
+    extern bool PC_KNOB_MODE;
 
     class PCGame : public games::Game {
     public: