Alias-free oscillators (#5826)

Add a band-limited, alias-free wavetable oscillator option to the
`Oscillator` class. Use it by default for Triple Oscillator.

Savefiles which do not have this feature enabled (e.g. old
savefiles) will be loaded without this feature to keep the sound
consistent.

Original author: @curlymorphic.
Fixed: @he29-net.

data/projects/templates/default.mpt

@@ -7,7 +7,7 @@
       <track muted="0" type="0" name="TripleOscillator" solo="0">
         <instrumenttrack pan="0" fxch="0" usemasterpitch="1" pitchrange="1" pitch="0" basenote="57" vol="100">
           <instrument name="tripleoscillator">
-            <tripleoscillator phoffset2="0" userwavefile0="" finer0="0" userwavefile1="" finer1="0" userwavefile2="" finer2="0" coarse0="0" coarse1="-12" coarse2="-24" finel0="0" finel1="0" modalgo1="2" modalgo2="2" finel2="0" pan0="0" modalgo3="2" pan1="0" stphdetun0="0" pan2="0" stphdetun1="0" wavetype0="0" stphdetun2="0" wavetype1="0" wavetype2="0" vol0="33" vol1="33" phoffset0="0" phoffset1="0" vol2="33"/>
+            <tripleoscillator phoffset2="0" userwavefile0="" finer0="0" userwavefile1="" finer1="0" userwavefile2="" finer2="0" coarse0="0" coarse1="-12" coarse2="-24" finel0="0" finel1="0" modalgo1="2" modalgo2="2" finel2="0" pan0="0" modalgo3="2" pan1="0" stphdetun0="0" pan2="0" stphdetun1="0" wavetype0="0" stphdetun2="0" wavetype1="0" wavetype2="0" vol0="33" vol1="33" phoffset0="0" phoffset1="0" vol2="33" useWaveTable1="1" useWaveTable2="1" useWaveTable3="1"/>
           </instrument>
           <eldata fres="0.5" ftype="0" fcut="14000" fwet="0">
             <elvol lspd_denominator="4" sustain="0.5" pdel="0" userwavefile="" dec="0.5" lamt="0" latt="0" rel="0.1" amt="0" x100="0" att="0" lpdel="0" hold="0.5" lspd_syncmode="0" lshp="0" lspd="0.1" ctlenvamt="0" lspd_numerator="4"/>